Power is an attitude, not a bunch of loot or money. A powerful person expects to be listened to, catered to, and obeyed. A powerful cleric is not going to wait around for things to happen, he's going to make them happen.
I think you're confusing Power with Agency.
You can have an enormous reservoir of power, but by being part of a structured society your agency can diminish rapidly. Being a High Priest you probably have a lot of power over those who are about... But how much power can you act upon given your current agency? There are new laws, written and unwritten, that you must obey at any given time. Adventurers are bound to a social construct where they are pretty much free to do what they will (or what their specific sponsor allows them to do)... This cleric now has to act within a bureaucracy. And bureaucracy moves sloooow.
Slainte,
-Loonook.