Because most people don't live in city centres where the "fun" actually is, so they have to get there somehow, sometimes from 10-20 miles away. They could all use public transit (in places where it's available), but that would increase the burden on that kind of transit. Especially for big cities, that was already quite busy - well, until the pandemic.

What I've seen happening here in the UK is that a lot of people leave their cars in a park & ride or close to a train station then shuttle in, which I guess is a step forward, but a lot are still driving all the way.