Just remember: the only difference between holding and PI of a Receiver is whether the ball has left the QBs hand.
So why is it only a 5 yard penalty with first down for holding, and PI is a spot foul? It is common for D players to hold a guy who is about to leave them in the dust and so they grab and stop the guy from proceeding.
And, there is no reason to assume that PI relates direct to stopping a player from scoring. Most PI's aren't in that category.
Spot foul is just too big of a penalty for a PI and it is too liable to simply be a play designed to cause a PI.