The purpose of the TSA isn't to catch terrorists. It exists a) to try to deter terrorists from trying to harm a plane, b) to make the flying public feel like they are doing something to make flying safer (whether it actually works or not), and c) to make the electorate think the US Congress is doing something about terrorism.
There is a lot of variability in airport security personnel. Nearly all whom I have encountered have been courteous, friendly, and fairly efficient. My daughter, however, encountered a female agent in London a couple of days ago who let a little bit of authority go to her head. My daughter had a properly contained bottle of special honey in her carry-on that cost her over $50. The label on the bottle said it contained 102ml, so into the trash it went and my daughter got a "severe talking to", and as a result of the ensuing delay she missed her connection to the US. Yes, I know, 100ml max is the rule, but most reasonable folks wouldn't have made an issue of it or held up the line in order to browbeat a passenger about it.