If you can make a game like Aggravation without calling by same name. What's stopping companies from doing it?

I was curious as to know what's stopping a huge company from taking someones idea and renaming it and not getting sued for copyright or patent infringement - if mechanics can't be protected? Other companies can make "Opoly" games without any worry of being sued by Parker Bros for basically copying Monopoly. So it doesn't make sense to me if that is the case.