The game. The wonderful thing about Adventures in Comaland is you can use it to explain away incongruences that typically occur in a video game.
Why do you only encounter a limited number of cars when normally the real world would be filled with far more makes and models? Because those are the cars Tanner's subconscious picked up right before he went into the coma. So those are the only cars he can think of while he's out cold.
Restarting missions after failure? Tanner wants to succeed, so his brain resets it back to the original attempt because of his motivation. He doesn't realize that's what's going on but we as players do, so we trigger it.
No one getting run over? Tanner's a cop, he has no interest in hurting innocent people so his mind always ensures everyone is safe if he drives over the sidewalk. They always manage to dive out of the way, even if a real person might not have those reflexes.
Crazy, improbable driving skills? Tanner idealizes himself as a movie-esque wheelman, allowing him to get away with more style and finesse than you could pull off in real life.
An unusual emphasis on cars, with most major plot developments involving them? Tanner's inner focus on driving causes his subconscious to make the world revolve around vehicles.
On a wider extend: This is also why Tanner has access to expensive cars. He has properly memorized some of his favorites or known quite a lot of cars during his time as a detective, which would explain the wide library of cars available. This also explains why his Dodge Challenger 1970 is the last unlockable vehicle but also the cheapest: It's his own car.