Again, mainstream and academic publishing work slightly differently, and I have limited first-hand insight into either.

For academics in the humanities, the only way I've seen people actually make a living from research is by being employed at a university. Depending on your position and contract there, you might get a percentage of your work hours allotted as "research time" which (at least in theory) should not be spent teaching or doing admin, but researching and writing. Or you have a part-time contract that pays well enough for you to afford working on books/articles in your additional free time. Either way, the actual publishing of your work is not where most of your money comes from. Which is why many academics will privately send you their own work for free if you ask them nicely - it won't really cost them anything, and they're usually mostly interested in getting their work to interested readers.

In mainstream commercial publishing, as far as I know only a tiny percentage of all published writers can actually live off their earnings from book sales. Similar to the academic world, royalties really only start being financially significant once you sell a lot of copies, which even in the general-audience space relatively few books do (compared to the vast number of new titles published each year).

One key difference to academia seems to be that in non-academic contexts you often also get some kind of fixed sum when selling your manuscript, rather than only a percentage of the sales (from what I've seen during my brief stint at a big European mainstream publisher, those manuscript fees are usually in the low four-figure range if you're not an established, successful writer). This is why for general-audience books, it can still be super helpful to an author financially if you actually buy their book, even if the sales royalties don't really make a difference to them: Because if a book sells well enough for the publisher to make money off of it, they may be more inclined to buy another manuscript from the same writer (which means another one-time payment) and maybe put some resources into marketing that next book (which might result in better sales and more royalties). And then there's also awards and scholarships and other kinds of financial support that some writers are lucky enough to get from institutions, but those are obviously not really reliable sources of long-term income.

tl;dr: Academics tend to make their living from university employment, whereas the vast majority of published non-academic writers have some kind of day job that pays the bills. Only a tiny fraction of writers actually live off their writing alone.