F-117 is old and getting unsuportable (though I suppose they could build more), the B-2 is mighty expensive to operate and more of a strategic solution, ditto with the B-1, with the added disavantage of not being a very stealthy design (redesigned intakes or not). The F-15C is more than good at what it does, but it's also getting old and it will eventually be unsuportable, and I doubt the Air Force would want to replace them with more F-15s. While the F-22 could be very easily be turned into an extremely effective strike platform, with only 339 of them to go around (or is it 200 now, I can't remember) they can't be everywhere, besides which they wouldn't be perfect for everything (CAS and FAC come readily to mind).
edited for grammar and punctuation