The whole system is being run by people primarily attracted to power over others.
The huge numbers of tickets are bought months in advance. By detecting the way they were bought (part of a tour package, through a tour operator, etc.) the people could be "screened" from any particular security screening at all.
Huge numbers of tickets are bought through employers, or their business travel agents and the purpose of their trip can often easily be discerned. Those people could be easily sent through reduced screening (and in fact we now have the "TSA Pre" program).
Large numbers of tickets are bought in groups of 3 or more because it is basically family traveling together. Terrorists don't travel in family packs.
Their efforts should have been focused upon travelers for whom their trips had no identifiable "purpose" (which would still be a large number of travelers). THOSE are the people upon whom SOME additional observation/screening/attention should be paid. And even then, it should probably be merely comparing them to the 100K+ people they have on their "watch" list.
As it is now, by watching "everybody" the TSA isn't "catching" anybody, except the guy that forgot his glock was in his gym bag.