It seems like almost every Hail Mary I see in College and the NFL has some level of PI but it is literally never called. I’ve seen much less egregious contact called on other plays.
Is it just expected and the referees think it’s too difficult to defend without PI?
I assume part of it being the design of the play is more of a jump play into a mass rather than a route and a defined coverage. And the fact that in theory it could usually be ruled uncatchable by most involved players, and also it may have something to do with the defenders all generally facing back towards the qb as well so they’re “making a play on the ball” which gives the defensive back the same rights as receivers to the path to the ball. While they could usually call playing through the back of the player, I think that can be negated by facing back often. You only get a clear path if they’re playing you not the ball. If they’re after the ball and knock you out of the way that’s a good play.
For example if the guy doesn’t turn to identify the ball and knocks you down while looking at you, pass interference. If that same guy turns and looks at the ball as he runs you over, no flag as he was trying to get the ball. Same end result of wiping out the receiver, but by looking for a ball they get the right to run that path, where if they face the receiver they will get flagged for taking that path over.