If 9/11 was an inside job, why...?

Would they go through all of the trouble to stage Shanksville and shoot a missile though the Pentagon? It just seems like extra effort that could get caught, so why not simply bring down the towers and maybe run an airplane into them? I'm not asking because I simply don't believe, because there is too much evidence of the contrary, I just want to know why they went through all the extra trouble, when it seems that bringing down the towers would have been enough.