These players represent multimillion dollar investments and GMs are held accountable for them. While some players who do have successful postseasons get generous contracts, I'd be willing to bet there were many other factors in play.
Pablo Sandoval is an example we can all identify with, as he had some terrific postseasons, and his deal certainly looked questionable (at least to me) at the time. But he was also just 28 years old, which was young for a free agent, and scored very well on a lot of defensive metrics. His post-season numbers certainly did not hurt him at all, but no one commits $90mill based solely on 167 PA spread out over 5 years. If teams did, free agency would have been kinder to David Freese, Steve Pearce, David Eckstein and Adam Kennedy...