Simplest formula of a calcium phosphate mineral?
Yes, you are correct in your calculation. However, the most common hydrated calcium phosphate in nature is indeed hydroxyapatite. Note that industrially, there are several other calcium phosphates (see link, note that none of them is your calcium phosphate) The reason that it forms instead of the "simpler" mineral is that it is more stable thermodynamically. I'm not certain about the exact reasons for that.

Apatite is so stable that in nature and rocks, almost whenever you have phosphate, it binds with calcium to form apatite. The only way to not have apatite is to not have calcium, not have phosphate, or have some really exotic system that suppresses its formation.

This is also known experimentally. In experiments that simulated calcium phosphate systems as early as the 60s, it was hard to crystallise anything but apatite.