There are many reasons: marketing propaganda, sexism, insecurity, politics, and selfishness. Americans are seemingly very susceptible to propaganda and marketing has convinced people that the type of products and brands we consume define who we are.
Sexusm. They have tricked us into believing that small cars are feminine and that huge trucks are for manly, rugged, outdoorsy types.
Insecurity. Even if someone realizes that judging people based on what vehicle they drive is stupid, lots of people are very insecure and are worried other people will judge them for driving a small car.
Politics. Leftists advocated for smaller cars because they care about urbanism, pedestrian fatalities, and the environment. In response, conservatives have turned large tank sized vehicles in a conservative political statement.
Selfishness. Engineering can only do so much to cheap physics so bigger / heavier vehicles are a little safer for the passengers of the larger vehicle when crashing into smaller cars but at the cost of the safety of the smaller car. It is far from a 1 to 1 exchange, large vehicles greatly increase the traffic fatality rates, but large pickup owners don’t give a shit about the safety of others. They have no love for their fellow countrymen which means they have no love for their country.
I agree with you and I would like to add a couple points:
Safety: a lot of people buy big vehicles to feel like they have a chance of surviving a crash with another big vehicle. Not saying this is good, but it is a thing that happens and it feeds the vicious cycle.
Regulation: I know these types of threads tend to lean into “regulation good” but let’s not forget that regulation caused (or a big part of it) this whole mess in the first place. Actually a couple of them: the chicken tax is a big one (a 25% tariff on imported light trucks), and the other one is the way the minimum fuel mileage requirements are calculated. Basically, you can break the math of the EPA fuel mileage requirement by making the vehicle longer. Unfortunately this looks aesthetically stupid, so it makes the proportions correct they have to be bigger in every other direction too. This mileage math is effectively what killed the mini trucks that used to be so popular like the S10, and smaller versions of the Tacoma and Ranger.