Because like most safety features, AM radio matters most in rare situations that people don't think about when they're purchasing a vehicle. If we let the market work it out we'll find that we lose access to AM radio because people won't notice until they need to tune in to a traveler information station and can't because their radio doesn't support AM.
Then we'll get a bunch of people who are either ignorant of the conditions or who are fumbling with their phones trying to find the information on the internet somehow.
We could phase in a new traveler information system that works in new cars, but that would require an enormous expenditure of resources to roll out the new system and then we'd have to maintain the old one for as long as there are vehicles on the road that don't support the new. It's far more cost effective to just mandate that all vehicles be able to receive signals from our existing system.
Why must all cars have AM radio?