They already knew that TSA screenings are security theater. It takes only a bit of casual observation, from the waiting lounge, to see which security personnel at which airport are competent or most overwhelmed. The 9/11 terrorists and any modern counterparts have opportunities to visit or travel through airports with _real_ security, like Egypt, Arabia, Pakistan, and Israel, and see the trade-offs between cost and security at all of them. There were compelling reasons that the 9/11 attackers chose Boston to fly from: one of the reasons was doubtless the very lax security at Logan Airport.
Doing a "lot of attempts" winds up getting one attempt noticed and tightening up security at that airport and possibly others, so "do a lot of attempts" breaks down quickly. What a lot of attempts could do is drain the budget of the TSA and of airlines: overwhelming the staff with hundreds or thousands of false positives over a day or a week would cost the TSA and the airlines many millions of dollars, Simply wiping or spraying enough nitrate residues on a an escalator handrail would expose hundreds, even thousands of people in a day to explosive detection and wipe out the resources and budget of many TSA offices.