Because Rooney is more influential than Nani. A lot more influential. Even when he's not at his best, you're more likely to get a positive influence on the game from Rooney than you are from Nani.
Saturday's game being a good example, with Rooney putting Hernandez through on goal with a brilliant pass and generally using the ball reasonably well (59/64 passes completed - 92%) in a game where the consensus seems to be that he was poor.
Nani, on the other hand, faffed about on the wing repeatedly conceding possession (30/45 passses complete - 67%) and generally being a complete waste of space. When Nani's not at his best his presence, if anything, weakens our team. The same isn't true for Rooney.
All of which means, in terms of really important, influential attacking players we've got Rooney >>>>> everyone else. Nani's probably just ahead of the pack but not by as much as some people seem to think. We're nowhere near as dependent on him as you'd think from some of the recent comments in this thread.
Jaysus, why does everything need to spelt out, piece by agonising piece, to some of ye lot?