1. For pleasure/curiosity/enjoyment (as in you are independently wealthy).
2. If you can't see yourself being happy in any other life and you understand the compromises you have to make (poor income, high competition, having to move a lot, etc.)
I know some wonderful PhDs who got doctorates with this very understanding and love their lives. But I didn't have the wealth to do it for fun, and I generally land on my feet and get happy no matter what, so I didn't borrow any more trouble and moved on with my life.
America's universities maybe may be different but this is how it was in Ireland!
The main problem with universities these days is that they are given the duty of both teaching and research, but they force people to teach that don't want to (some professors, grad students). In addition many people view universities more as job training than learning about a specific subject. If you go to college to get a degree in CS, but all you want to do is get a job at Google, you don't need CS experts to teach you intro to data structures or how to write SQL.
I think the long term solution is to entirely separate the realms of research and teaching, or to make teaching as a research professor entirely "opt-in" outside of discipline specific classes (the algebraic graph theory professor should probably be the one teaching algebraic graph theory). My university has been relying more and more on teaching-only professors to teach CS classes and I think it's a wonderful solution. The teaching-only professors are much more personable and engaged in the actual teaching, and the research professors get to focus on what they're good at.
No. PhDs are extremely specialized on a very narrow topic within a single field. They spend years researching that topic to the exclusion of everything else.
Asking PhDs to teach is like asking brain surgeons to treat common cold. They probably can do it, but it's a gross waste of their skills and expertise.
The result is an educational environment where the most visible job requirement for faculty is to teach courses in their field. But that is the portion of their responsibilities for which, from an incentives perspective, they are least accountable. Instead, to obtain a tenure-track position and ultimately earn tenure itself, primary requirements are research and publishing output, with double++ good to eleventy for bringing research grants to their institution. Teaching and advising/mentoring, not quite an afterthought, is much lower in priority & weighting in tenure review.
I have seen faculty in tenure track positions penalized for focusing too much on the side of student instruction, advising, etc. In one particular case tenure was denied on the basis of a light research portfolio despite an extensive and impressive track record of well-rated course instruction and an impressive dedication to meeting with students far more the the minimum required time. (2.5 office hours/week) Of course there may have been more to the denial, I only know the publicly available details from the fallout when a dispute was filed.
Why?
Research on the other hand, outside of the self-serving aspect - like how many papers one gets published - has hardly any immediate benefits relative to the benefits of teaching... and few long term benefits. It's more of a gamble.