you don’t want a good hitter prematurely breaking down because he spends 130 games a year getting the crap beat out of him. Essentially anyone who is a good hitter is too valuable to play as a catcher. hence why most catchers are good at something other than hitting. The good hitting ones got moved early on. Posey just happens to be a freak (I think unicorn is the en vogue term) who is so good at everything behind the plate that moving him would cost you more even if his bat got better.
We might be seeing that very thing (premature breakdown) happening to Posey though. Last year was by a wide margin his worst at the plate (116 wRC+, .341 wOBA, .147 ISO). It will be interesting if that was a one year blip or the start of a decline and expedite the long time calls to move him out from behind the plate.