Why do people think that 911 is a conspiracy?

a lot of people think that 9/11 was a conspiracy because of all the videos of evidence that 9/11 was a controlled demolition experiment by the government. the planes were never found and bodies came to hospitals of people with no names also with amnesia. there has been proven evidence that assassinations took place during the dark hour of 9/11. the victims of the murders were government officials even building workers. i have seen two videos that stunned me enough to answer this question. one video was that what looked like nothing but an explosion and no plane but when put on t.v. an orb came up that looked like a plane or missile. the second video showed construction workers on phone when a random explosion came from in the world trade center. i have done the math and science and a plane CAN NOT just explode when in impact.