Not always. My wife is an adjunct at two different colleges. She would love to be full time, not just for pay or benefits, but because it would mean that her opinions and class proposals would actually be taken seriously by the department.

As for why she does it?

>> Sunk cost fallacy

I'm assuming this is based on the phrase "those who can, do; those who can't, teach". That all of her time has been teaching, so she might as well keep teaching. I'm also assuming you've never met someone who is actually passionate about their work. Is willing to work at 70% salary because funding is tight, or will take a pay cut to do something they love versus something they have little emotional investment in.

>> 2) What else are you going to do when all your training is academic?

I'm assuming here that you've never talked to anyone concerning their experience in getting a PhD. While many go on to academia, very little do it specifically to teach, many enjoy the prestige, pure research communities, and subject matter they study.