Run draws defenders into the box and then there is more 1-1 coverage for the passing game. Just the opposite of what you are saying.
Also opens up play action.
Which is not all that helpful if the majority of your passing game is compressed (suppressed) to that box (because your passing game is poor) and you do not run play-action. In modern football, you have to be sufficient in passing. In reality, the lack of a passing game has just taken something we should be good at, and made it mediocre.