Why do NP-complete problems not have similar approximation ratios?
Reductions are defined with respect to the decision version of the problems.  Approximation ratios for their optimization versions are a separate question, which seems related but doesn't necessarily have to be.  So to answer your question with a question, from a philosophical perspective, why should you expect the class NPC to preserve approximation ratios when it isn't defined with respect to them in the first place?