And "probably" doesn't equal "certainly." It's certainly within the realm of possibility the TSA's screening methods are ineffective.[1] But given the 16-year perfect record[2] in preventing not only hijackings but also bombings (which cockpit doors, air marshals and bathroom line policies are powerless to prevent), it's only reasonable to say their methods probably have some effectiveness to them, which (repeating myself here) is not to say they're the best possible screening methods. And there are most certainly people who think there's "reason to" attack US airplanes, hence the foiled incidents (shoe bomber, liquid bomb plot, underwear bomber) that engendered the more annoying TSA screening policies.

[1] Although I'd say the more likely problem is that the personnel doing the screenings may be ineffective, rather than the fairly-standard-worldwide screening methods they use.

[2] Which in all honesty is quite impressive. Immediately after 9/11 I'd never have guessed we'd go 16 years not only without another 9/11-scale attack, but no attacks on airplanes at all. It's not yet as good as Israeli airport security's 41-year perfect record, but it's nothing to sneeze at.