Certainly on could create an RPG where monsters don't have ability score, but to remove them from 5e would take a ton of work. Ability checks and saves are just too big a part of the system. Cast fire ball on them? That's a Dex save. Want to grapple them? That's a Str check. The list goes on and on. You'd have to either replace those with something else or alter how all those mechanics function.