Why are great prophets the only GP that have the same faith cost counter?

I like hoarding faith, but it looks like the game has a tendency to buy prophets for you if you ask it to save for later. Which is where the Holy sites would come in handy as you're just essentially reinvesting the faith.

You do get your faith back by planting your 500 and 800 faith prophets + the free ones from piety finisher and Hagia sophia, but after that--not really. Messiah belief + freedom new deal might let you squeeze in one more. If you're running a really high faith game, you're better off just using the 1200-faith prophet (1 prophet can usually convert and fill two CS quests, on cities with other religions really entrenched), or buying other stuff like holy warriors or missionary spam. You can buy quite a few warriors for 1200 faith.