1. Stricter safety and emissions regulations that make new cars more expensive to make.
2. Supply chain issues making new cars more expensive.
3. New cars are expensive so people are holding onto their used cars longer, shrinking that market.
4. Cars are much more computerized than before, so it's hard to fix up old cars and make them work again.
5. Teens used to be able to take auto-shop and learn enough to fix up an old shitbox to use for their first few working years.
6. It's hard to find an old shitbox anymore, even one that is easy to fix, because people are holding onto them until they die.
I'm not sure what the solution is here. The safety and emissions regs are there for a reason, but those are what lead to all the other problems.
We just need to design our society for fewer cars, but that change will take a while.