Those aren't exactly the same thing, are they?
The existence of gods is a fluff element. It's just there. The existence of psionic characters is a fluff element - it just so happens that there are psionic characters.
But those aren't rules.
Things that do need to be justified are complete new subsystems. Like psionics, like incarnum, and like binding.
The existing magic rules are not good rules, but they wouldn't be any worse at portraying psionics than they are at portraying magic. However, they would suck even more horribly at portraying a totemist or a binder.
That's why incarnum is justified in the fluff and why binding is justified in the fluff, but psionics aren't.
The psionics system works as a variant magic/psionics system, and it does it very well. But the differences between magic and psionics aren't significant enough to justify its existence.
Simply put, psionics and magic are similar enough that one subsystem could have been used for both, so using two is just bad design.