The game records those as 2 different days btw
You're contradicting yourself here...
If I log out of the game, save, and log back in, according to the used car dealership, a day hasn't passed, so why can't I do that?
Also, a real life race car driver can't pause in the middle of a race.
A real life race car driver can change his oil between races.
A real life race car driver can change his uniform somewhere in his lifetime if he wants to.
A real life race car driver doesn't take longer to wash his car or change his oil than to upgrade his turbo/engine/ECU.
A real life race car driver isn't limited to 20 million dollars.
Frankly, I'm not convinced this was a "decision" to make the game more realistic. It was probably just a big mistake, and we're the QC department.