The reason this happens is because of a flaw in the passing game. If it wouldn’t be for the defense’s miraculous tips and speed bursts and eyes in the back of their heads, then a player’s completion percentage would be unrealistically high, because the QB’s in the game are too accurate with their passes (If you’re throwing to someone in the passing cone of course). If the game took into account that many of the incomplete passes in the NFL are actually because of errant throws by the QB, where neither receiver nor defensive player can catch it, then there wouldn’t need to be this “cheating” by the defense. In the end, I believe it cancels out and you are left with a fairly accurate completion percentage at the end (can’t say the same for the computer’s QB though).
I personally would much rather have more randomness with the passes and less B.S. plays by the denfenses, but what can you do?