A huge chunk of it is because the USA has a huge tax incentive for car manufacturers to make bigger cars. When fuel efficiency standards started coming in, trucks were exempted because farmers needed their trucks for farm work, it’s a loophole that encourages the manufacturers to build bigger vehicles to avoid these taxes. These massive vehicles are unusually cheap in the USA. If these loopholes regarding fuel efficiency were closed out people would be financially incentivised to buy smaller cars. Unfortunately, money talks. People aren’t all selfish, they’re just doing what makes sense for them.