Evil can act Good, but Good cannot act Evil. Evil has that advantage over Good that cannot be matched.
This continuously plays out in our political systems. Good actors cannot fight the deluge of evil of their opponents, who are free to lie cheat and steal.
Good binds its own hands. For good to win, it must act with even greater evil. At which point, it becomes the very thing that it tried to defeat.
The assumption here is that being Evil is advantageous. It's obviously advantageous to personal gain, but the relevant thing is whether it's advantageous to staying in power, and then the system you want to design is the one where it isn't.
Notice also that it isn't binary. You can make it harder to be Evil and then have less of it even if you never have none.
Rarely a great RPG forces my good party to make difficult decisions.