Probably because it doesn't work for everyone. Working closely with someone requires skilful communication and professional maturity, it's not as easy as the books made it out to be.

I was lucky enough to work in a serious PP company for two years, paired with ~10 people. I can appreciate PP when it's the right person, but when it's not, it can be downright unproductive.

A lot of software engineering practices like PP, TDD were preached like some kind of gospel but never stood the test of time.