Maybe because you could give the account/game to someone else...
I would appreciate it as well if we could have several drivers/characters, each of which could re-do everything... It would be fine if you could win only one summit vehicle, this could be shared for all drivers (same for each vehicle bought (directly or indirectly) with CC), maybe even with an option to transfer the other cars from one driver's garage to the other's. This would also solve the problem with characters...