they don't. Look dog, the cars they get are sometimes over 15 years old and have ran like 200k miles. Those things are falling apart. You can make it nice and shiny and put a lot of expensive aftermarket stuff in it, but WC customs isn't fixing your engine, suspension, chassis whatever if it's all fucked up. Well, perhaps you get new springs if your car is getting lowered, but on a technical level, they're not fixing anything really.
basically, they make the cars nicer, not better