Because when your car wears out, and you buy a new one, you don't want it to have the same known problematic issues. I'd want the new one to have been improved.
Most brands do offer the upgrade parts. I upgrade and string along old cars for a while. Think my ebuggy is 4 years old.
Likewise, the manufacturer doesn't want you jumping to another brand because of known issues that they can fix in the kit.
Remember how we all switched between AE and Losi every 2 years? Not necessary anymore.