First, they don't. There's a HUGE CS faculty shortage. CS faculty positions outside of R1 are insanely hard to fill and the average quality of faculty tends to be quite low. Some lower tier colleges and universities will hire tenure track faculty out of online masters programs with no teaching experience. Even then, faculty searches regularly fail. The bar is rock bottom. (This isn't meant as a bias against online programs; the salient point is that if your masters was online then you almost certainly have zero in-person teaching experience even as a TA.)

Second, most people aren't rational economic actors and even those who are have a myriad of objective functions.

Some common reasons people stay when they could get much higher paying and lower stress jobs:

1. Immigration.

2. Work life balance. Not unique to academia, btw. I know ex-FAANG SWEs who make $70K/yr because they can get the job done in one or two days a week from a van and want to spend most of their life in the mountains.

3. Immigration.

4. Some people actually enjoy teaching and view it as a life calling.

5. Immigration.

6. Ageism. A 50 year old who spent 20 years in academia is going to have a hard time getting back into industry.