The dice are an illusion. They only impact the game in as far as they determine the result of an action, and both types of action only have two possible results.
Rule simplicity is also a factor, because they want the game to be easy to learn. Grapple checks are one of those weird exceptions to the normal flow of combat which is not supposed to ever really come up. They need a rule for it, just in case, but it's too obscure of a situation to warrant defining a "passive Athletics" stat like they did with passive Perception.