That doesn't make sense. Why should we have a thing that is rarer?
Because the amount of tech needed to repair a car could be too much or too heavy compared to a bike.
The fact that cars are so common would make it boring, it's not GTA, you shouldn't be able to just hop into any car you find just because you have a fission battery and a few tin cans. Not to mention the Bike would handle the terrain a lot better than the cars with less repairs needed.
Logically in the real world it doesn't make sense but for the game it's perfect.
Motorbike > Car
2 wheels > 4 wheels
2 seats > 5 seats
quick jumping on and off > opening and closing doors
No seatbelt > Seatbelt
Less likely to hit potholes > twice as likely to hit potholes
Faster > Slower