My guess is that car makers have decided that minivans are the best solution (for them) for those who regularly need to seat more than 4 people. I imagine they can charge more than for an "extended" car, and that minivans are generally classified as light trucks and thus have relaxed emissions standards. Some of this is tied to consumer preference but I bet a lot of it is really driven by our regulatory approach, or lack thereof.

I think I prefer this look to the sliding door you usually see on minivans.

I wouldn’t call it a car, but the GEM e6 has this design, and is a very direct extension of the e4

My first thought is to wonder how 6-door cars in this form factor would perform in crash tests.

Jeff, I wonder if there is a safe way to add a 3rd seat in the front row. This would solve the extra door problem. Cars used to have front bench seats. I know they were unsafe.

I thought you explained this pretty well already in https://www.jefftk.com/p/the-cost-of-a-sixth-seat, or do you think this is a different problem?

In the late 1960s I knew a family that owned an 8-door station wagon: we lived in the same town at the time, and one of their 20+ children (all full siblings) was in my class.

My family moved to a different town 30 miles away in 1971. Five years afterward my high school Spanish teacher mentioned that his wife's uncle's family was very large and came to family reunions in an 8-door station wagon. I asked some questions, and learned that it was the same family.

I do not know whether their car was built that way originally or whether it was an aftermarket conversion.

I also do not know what safety features were built in. Seat belts for all seating positions were mandated starting in 1968, but the car may have been built before then.

This is reminding me of the idea of something like an SUV having a roof which could slope down (like a hatchback), if there were only people in the front seat. (Or would be flat like a regular SUV, and fit more people). Which should reduce drag, but would presumably be really difficult structurally.

I doubt the lack of 6-door cars has much to do with aesthetics. Doors and tight door seals are some of the more complex and expensive portions of the car body. Doors also pose challenges for crash safety as you have a large opening in the car body weakening the main body's structural integrity in an accident. I suspect that the reason there are so few cars with 6 doors is the extra cost of manufacturing cars. That would lead to increased car costs. Most purchasers don't value the extra convenience of the additional doors enough relative to the added price. Any company producing such a car would find a very small market which might make it not worth it to the manufacturer.

Long cars may be difficult to park, if most parking slots are the size of the average car. (No idea how much that applies to USA.)

That might be an argument in favor of your proposed "six-door but not too long", unless it is the case that any car with six doors is already too long and the extra few inches don't significantly change the outcome. Like, you get less space for luggage, but finding a parking place remains nearly impossible anyway.

(My wife loved having a tiny car while we didn't have kids, precisely because parking it was extremely easy.)

What if it was just one big door? That would treat the problem J_Thomas_Moros raised. It wouldn't work with a regular door, it would be too long and awkward. I don't think it would work well with sliding doors. But, I think it can work with doors that open upwards, like the Tesla model X:

In most situations where people need more seats, those seats are primarily for kids, not the people buying the vehicle or other adults, and that makes the issue of every row having its own doors less important. Your kids just get used to climbing over and around things, and it's no big deal. The people consistently buying vehicles for more adults are mainly fleets buying vans and limos.

Also: I know you mentioned this in the original post in terms of car vs SUV, but adding length, doors, and weight will drop fuel efficiency, which may cause legal issues. Maybe not in the US, IDK, but car company platforms are global, and other countries have much stricter standards. This may make it just not worth it for what they expect the market size to be.

I wouldn't be too surprised if we started seeing vehicles like that in the 2030s though, once self-driving vehicles become common enough (I still think it's unlikely, just much less unlikely). People and companies have been talking for a long time about how much design freedom not needing a driver gives you, and how you can make reconfigurable interiors. Since it's likely that ridesharing and other fleet vehicles will make up a larger proportion of cars in an autonomous vehicle world, it's much more feasible to have a few vehicles around for less common use cases. Last-mile delivery automation product developers have come up with a lot of concepts that include vehicles with multiple independently locking compartments; it's very plausible a six door vehicle of some sort could reuse such a platform.

We had a wood-sided station wagon when I was a kid. There were six kids- myself, 2 sisters and 3 foster brothers. I remember those two seats in the back. I'm sure they weren't very safe, and it's not like cars had seatbelts yet. Oh my! I sound ancient!

How is a minivan not the optimized solution to this problem? One long sliding door, 3 rows, good fuel efficiency and safety and handling. And a plug in hybrid. It's what waymo went with.
.I am not saying it's the best possible but it's an optimized design to solve this problem.

I don't think any of the answers I've seen here or on LW can be right, although I find it interesting that the very implausible answers showed up in both places. I wonder what makes these answers seemingly compelling when they seem (IMO) not even remotely plausible.

Cost? If doors were so expensive, there would be a significant difference between the cost of 2-door and 4-door cars of the make & model and there generally isn't when you can price out a similar configuration (and more cars would copy the Hyundai Veloster design, which only has a rear door on one side).

Also, cars have more than an order of magnitude variation in price but a very small variation in the number of doors. You can get engines that have nearly 1000 HP, cars with sub 3s 0-60 time, car interior options that add more to the cost of a car than the MSRP of an economy car, etc., but somehow adding two more doors is so expensive that it's impossible to do? You can buy cars that have plenty of features that are useless from a practical standpoint that cost a lot more than two extra doors.

Crashing testing basically has the same problems as the above (you'd expect to see a difference in crash testing in existing cars) and, on top of that, if you look at the crash tests that are actually done (IIHS, NHSTA, Euro NCAP), there shouldn't be a major difference in scores with the addition of more rear doors (the one exception to this might be Volvo, which runs a number of crash tests that aren't done by testing agencies, but they're a niche manufacturer that has very low marketshare and that can't account for this).

Additionally, you could use a door the size of the door on a 2-door car and have a 4-door vehicle with better rear seat access.

People really seem to be underestimating the impact of aesthetics. Automotive trends are frequently driven by aesthetics (e.g., the trend towards larger wheels with shorter sidewalls, which is very close to being strictly worse than having 90s-height sidewalls, which give you better fuel efficiency, lower cost, better acceleration, better braking, higher resistance to wheel damage, and a better ride, at the cost very slightly slower turn-in; I don't think tiny sidewalls really make sense for non-aesthetic reasons on any car except maybe for the Civic Type R, and even there, it only even maybe makes sense due to some really unusual design constraints on the vehicle).