Meggan draws her power from her empathic link to nature, Storm has to manipulate preexisting weather patterns, telepaths draw power from the astral plane, Scott needs sunlight, Havoc needs cosmic energy, but the fact that they and a lot of other superbeings rely on a outside sources is never questioned. Why is that?
Jean is still an omega level mutant, the only difference between her and other marvel psi talents is that she does not have to exclusively draw power from the astral plane, she has a back up source (the Phoenix Force) due to her having reached her omega level potential it is a her was ultimate mutation she evolved into the Phoenix, Death told Jean way back when that her unique power was to wield the Phoenix. Jean is not a host of the Phoenix, she is the avatar there is a huge difference. Jean is the house where Phoenix lives, Rachel only got a portion of the Phoenix she's the apartment where the Phoenix sublets. The Phoenix was drawn to her because her astral form had a psi signature that was almost identical to Jeans. The Phoenix has never chosen anybody else to manifest through if Jean is around and not denying that part of herself. Nate manifested a Phoenix raptor once while high up in the earths atmosphere, it may have been a psi construct of his own doing, but considering he can't handle his nigh-infinite powers without them killing him, I don't think connecting to something that amps power to the level the Phoenix does is a good idea. Maybe Cable was never picked because Jean and the Phoenix don't like his real mommy.