They don't think disasters are likely. On the other hand, I also don't think disasters are likely! While we have extra water in the basement, I think the chances we'll need it sometime during my life are only maybe 2%. Since it's not expensive, and if we do need it we'll be incredibly happy to have it, I think it's worth setting up.

It does matter a lot whether the chances are ~2% or 0.0002%, but if you think your lifetime chance of being impacted by a serious disaster is under 1% I'd encourage you to think about historical natural disasters in your area (earthquakes, floods, hurricanes, wildfires, etc) plus the risk of potential human-caused disasters (nuclear war, epidemics, civil war, economic collapse, etc).

It's weird. Most people don't do it, and a heuristic of "do the things other people do" is normally a pretty good one. In this case, though, I think we should be trying to change what's normal. The government agrees; the official recommendations involve a lot more preparation than people typically do.

They can't afford the money, time, or thought. Many people are in situations where planning for what's likely to happen in the next couple months is hard enough, let alone for things that have a low single digits chance of happening ever. This can't explain all of it, though, because even people who do have more time and money also haven't generally thought through simpler preparations.

They don't think preparation is likely to be useful. If there's a nuclear strike we're all dead anyway, right? Except most disasters, even nuclear ones, aren't this binary. Avoiding exposure to radiation and having KI available can help your long-term chances a lot. Many disasters (nuclear, earthquake, epidemic, severe storm) are ones where having sufficient supplies to stay at home for weeks would be very helpful. If you think preparation wouldn't help and you haven't, say, read through the suggestions on ready.gov, I'd recommend doing that.

They're used to local emergencies. We generally have a lot more experience with things like seeing houses burn down, knowing people who've become unable to work, or having family members get very sick. These can be major problems on a personal scale, but families, society, government, and infrastructure will generally still be intact. We can have insurance and expect that it will pay out; others in our families and communities may be able to help us. Things that affect a few people in a region or community at a time are the sort of things societies have the spare capacity for and figure out how to handle. A regional disaster works very differently, and makes planning in advance much more worthwhile.

They expect to see it coming. Forecasting is good enough that we're very unlikely to be surprised by a hurricane, but for now an earthquake could still come out of nowhere. Others seem like the kind of thing we ought to be able to anticipate, but are tricky: it's hard to see an economic collapse coming because economic confidence is anti-inductive and we tend to suddenly go from "things are good" to "things are very much not good". Paying attention is valuable, but it's not sufficient.

They're not considering how bad things can be. For many of us our daily experience is really very good: high quality plentiful food and drink, comfortable and sufficient clothing, interesting things to do, good medical care. When you consider how bad a disaster can be, things that would improve your life a lot in very rare circumstances can make a lot of sense.

They're not sure what to do. This is pretty reasonable: there's a ton of writing, often aimed at people who've gotten really into prepping, and not much in the way of "here are a few things to do if you want to allocate a weekend morning to getting into a better place". Storing extra water (~15gal/person), food (buy extra non-perishables and rotate through them), and daily medications, however, goes a long way. For a longer list, this guide seems pretty good. (Though they're funded by affiliate links so they have incentives to push you in the "buying things" direction.)