>Why does everyone say this?

Because the most popular theories about 9/11 conspiracies are never as simple as you outlined. I think a small group did plan and carry out the attack, and a part of the US government is just as capable of doing that as Al-Qaeda.

But once you start adding extra details (the buildings had explosives, the pentagon got hit by a missile, etc), the number of people you need "in on it" starts to climb. That is why so many people point out the huge headcount of the supposed conspirators.

The conspiracy theories generally hold that 9/11 wasn't perpetrated by "terrorists", regardless of their benefactors. E.g. WTC towers were brought down by explosives, not planes crashing into them, Pentagon was hit by a missile, not a plane.

The simplest and most plausible theory is simply that Bush and Cheney knew, but didn't do anything because they needed the attack to go into Iraq. Even then, at least tens of people in the White House and upper echelons of US intelligence would have known and would have known that they could have stopped this by making a few phone calls. Even 12 years later, not one of them has regretted his complicity in killing thousands of civilians and spilled the beans to a reporter.

I think whether the government was involved in 9/11 is irrelevant. Seriously who cares? I don't think the American public even cares. How do I know this? A simple example is the Iraq war. One doesn't need to be a genius to understand the immorality and illegality of that war before it started. After it became fact that the "reasons" for starting the war were complete bullshit, what did the American public do? Nothing, we did absolutely nothing. Patriot act, escalation of the Afghanistan war, drone strikes killing innocent civilians, targeted assassinations of American citizens, indefinite detention, etc. All these actions, and more, by government are just as bad or even worse than 9/11.