Well I'd say why would the government do half the things it has done (or continues to do) to citizens (and citizens of other countries)?
Because quite frankly they just dont give a toss one way or another about you or I, so long as their power structure remains and remains in good working order, or otherwise if they get the chance to maximize or enhance their power structure, then 3000+ american lives, plus a whole host of other lives here and there, is not too much a price to pay, and in fact is quite small in scale to them im sure..
in short, why would they destroy the twin towers and building seven? I cant say why 7 was brought down precisely, there are some theories...but WTC 1and2 was for pure shock value, so that we would go along with what they had planned for later. Nothing like 3000+ dead americans in one or two hours to light a fire under peoples asses, or make them so fearful that they will literally HAND you all the power you've always wanted to have openly, but never have.