One of the issues is reputational. Trad pub sees itself as a cultural linchpin - the gate-iest of a gatekeepers, a setter of major trends.

From one POV this is nonsense. Trad pub throws a lot of books at the wall and a few of them stick.

But from another it does so selectively. In fiction the big advances go to established or potential personalities - not so much to outstanding authors, but to authors who are known to sell well.

For new authors that means trad pub looks for individuals who will appeal to a target demographic. (For new contemporary fiction that usually - but not exclusively - means aspirational, college-educated, female.)

Which being the case, even limited PR is better than no PR. And the big pubs can do effective PR in a way that solo self-pubbed authors can't, by setting up reviews/interviews in the mainstream press.

An interview or a review typically costs nothing, but can be a huge driver of sales. The author needs to be reasonably interesting, at least a little photogenic, and have some kind of personal story the demographic can identify with and maybe admire. (Not usually the same story as the one in the book.)

So it matters who does this, because it's not just about the money. It's really about a monopoly on gatekeeping cultural status.

The money takes second place.

Which is why publishing is simultaneously almost comically amateurish but also throws big sums around. The amateurishness is a remnant of the days when there were tens of medium sized publishers run by amateurs and enthusiasts who would often publish books just because they liked them.

The industry is much more of a corporate monoculture now. But clearly it's still better to hang on to some remnants of choice and diversity - even if the choice is between a handful of monoliths, each of which still has a unique culture of sorts, instead of tens of smaller houses.