Most, if not all, manufacturers have required as part of the licensing agreements that their cars be able to finish a race. Ferrari does not want to have the engine separated from the monocoque of it's Enzo, Ford does not want the engine intruding into the passenger compartment of the Focus, and Nissan does not want to see a GT-R wrapped around a pole.
Yes, the damage could be a lot better visually, but if you want real cars, you have to live with their licensing agreement, and the restrictions to damage.