I always wonder about that. Often, in the line at the boarding gate several agents will walk around, compare your boarding pass with your passport (and your face), and then draw a squiggle on your boarding pass (sometimes with a coloured felt-tip pen, sometimes with a biro/ballpoint pen).

It seems to me that it would be trivial to squiggle on your boarding pass yourself, and then claim that you've been checked already. I wonder how much security theatre is happening there, too.

But usually when people get to the front of the line they still present both documents, the fact that 9/10 times the passport is ignored just makes it a judgement call by the ground staff.

Having spent some time working on staff management systems in airports I can say with some confidence that (at least in australia) most of the ground staff will immediately flag someone not at least offering their passport, and/or trying to talk their way out of needing to do so as sus.

And let's not forget that if your entire plan was to get on a plane under a fake name, it's a hell of a risk to just hope that you end up in a situation where some chap is squiggling on boarding passes.