Let me ask you guys a question

If you can see a pitch is low, yet the catcher moves his mitt up into the strike zone AFTER he catches it, why does the umpire call it a strike? Can he not see the catcher’s mitt moving up from the low spot back into the strike zone??? The batter’s getting robbed if he’s calling those pitches strikes when it’s clear the catcher is moving his mitt back into the strike zone. And no one’s calling the ump on it?? It’s friggin obvious!