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.

Because leisure time often coincides with outdoor activities and traveling to other places. The experience is much nicer when you're outside of your city. And even in Europe there're many places to visit which don't have train stations.