I know for absolute fact planes hit the towers on 9/11. How? I saw them on live tv. Watch enough tv, film, and video and you can easily spot the differences. CGI looks like CGI, live tv looks a certain way, recorded tv looks another way, film looks like film, etc. People's reactions too confirm reality or fabrication. Body language, tone of voice, where the eyes are looking.
There's no way they faked 9/11 or it was anything other than planes and terrorists.
People disputing it are like religious lunatics refuting science or perpetuating young-earth hypotheses. They take some initial eye-witness account (which historically are almost always wrong) then run with it, facts be damned.
You could never keep a deliberate government-backed act to do 9/11 secret. Someone's always willing to be martyred to get the truth out and would come foward with irrefutable evidence.
The "evidence" of conspiracy types isn't any kind of evidence unless you broaded the definition to include eyetwitness testimony and hearsay.