They are not exactly getting paid for writing, they are getting paid for their domain knowledge and certification. In the same way you could say devs get really well compensated for typing.

I'd think that most writers who get paid are writing about some domain knowledge. This would seem to be true of fiction writers, even if the domain is an understanding of human relationships, insights about sex or ambition, etc. Otherwise the pay is likely to be very poor.

A lot of their writing gets done by people making less than them, like legal aids. I've written content for lawyer's websites for not much money.

I also wrote legal notices as part of my corporate job. I started at like $9/hr and was make $12-something/he when I left.

Some letters got reviewed by the legal department and approved by my boss, but I wrote them (using a form letter to get me started, then fill in the specifics).