I can imagine a lot of possible reasons. First, Cheney really believed they'd be there, and Bush figured that if a smart guy like Cheney believed it, then they would be there. So, they didn't have a Plan B.
Second, that Bush and Cheney had a certain code of honor: spin, embroider, and cover-up, sure, but don't plant evidence like a dirty cop.
Third, that organizing a hoax would be hard and risky.
Fourth, that U.S. soldiers have too strong a sense of honor to go along with a massive hoax.