"In most cases, we wouldn't hear about it if it does happen. A guy making millions isn't in a rush to admit that he got hurt clowning around. Some of those pile ons get nasty looking."
"Nasty Looking" is the key here, it's not like if the guy being celebrated is really trying to get away when they tackle him. The first guy to reach him wraps his arms around him and he goes down slowly and then they all crowd around and slap each other on the back.
Imagine if we saw how these guys horse around in the hotel pool, or in the locker room? Celebrating after a win is just part of the game. Sure once in a blue moon someone will get hurt but, again that can happen doing just about anything. I think more players get hurt shagging balls in batting practice then they do in victory celebrations.