It's a social thing!
When was the last time you pulled your car over to talk to a friend driving the other way?
Riding does a lot of good for me. Both brain and body. My favourite part of a working-day is often the rides to and from work. It gives me time and space to think, and it feels meditative. In the morning it feels like it wakes me up and primes me for the day. It gets my blood pumping. In the afternoon it's the delineation between work-time and home-time. But really it's not the best bit of the day.
The best bit is the social aspect of riding a bicycle. I don't mean the group rides, necessarily, although they are pretty nice too, assuming you have the right group of buddies to ride with with a good destination. What I'm referring to is the random bump-into of friends that you can have when riding. You spot someone riding up ahead that you recognise so you chase them down to say hello and ride together for a while. You spot someone walking on the footpath so you do a hasty u-turn to stop and have a chat. Or just a pull-up in the middle of the path for a chat. Or someone has a really cool bike so you stop for a chat and some mutual bike admiration. It happens often and I love it.
I won't say that this never happens when driving, because I'm sure it has happened at some point out there. But it's uncommon. The number of chats had with other drivers, as a percentage of trips, would be very close to zero.
Yet another reason why bikes are better.