Why do people like to think that planes didn't hit the WTC and Pentagon?

When there is video clearly hitting both. For those who think Pentagon was hit by something else...a href="http://www.youtube.com/watch?v=YVDdjLQkUV8" rel="nofollow"http://www.youtube.com/watch?v=YVDdjLQkU…/a I am not on here to troll, I am asking a serious question and want a serious answers from people who aren't 13 year old kids on youtube...thanks.