So I think that's true for some roles but not all. I personally end up arguing about what markets we should and shouldn't enter, presenting at industry conferences, etc., because I find that interesting. But, for instance, a junior ops employee on a pager schedule primarily provides business value by getting good at the technologies that other people have chosen, not by paying attention to the market, by being able to present convincing arguments to others, etc. Yet that's a crucial role to business values.
I think this may be true of junior roles in general, and I think it's junior roles where I'm most worried, as a social-responsibility thing, about the fairness of salaries. I care less about earning $150K when I should be earning $200K than I do about earning $50K when I should be earning $100K, since it makes far more difference in my quality of life. (Even though they're the same delta value for the company!)
I could buy that there is a more generalize skillset of communication and persuasion: ops getting dev to clean up messes before putting them in prod, developers convincing open source maintainers into fixing their bugs for free, employees convincing prospective new hires to join, etc. And I do think it's valuable to measure that in addition to raw technical skill, both in hiring and in performance reviews. But I suspect there's enough reason to doubt that that correlates strongly with personal salary negotiation. Some people just don't do well pushing back on their direct superiors, but none of these other cases involve direct superiors. Somewhat more rationally, if you're worried about your job for office-politics reasons, or if it's riskier than usual to lose your income (maybe you're starting a family or have health issues), the right decision might be to take a safe, lower salary instead of trying for a higher salary with a small percentage chance of losing the negotiation badly. People without those concerns -- like me, who lives alone with no medical expenses and can bother my parents if worst comes to worse -- have a much higher risk tolerance, and I don't think personal risk tolerance particularly proxies for business value.
I also think that skill is learnable, and there are obvious poor incentives for a company to teach employees how to ask for more personal salary, but probably great incentives to teach employees how to negotiate better in the general case.
(As I understand it, big law firms rely on a sizable work force of associate lawyers and especially paralegals and other staff, who at least start out with very little say in the direction of the firm, what cases are taken, etc., right? And the partners are owners, not simply employees? I can definitely imagine open salaries and probably even unionization for the staff, which wouldn't compromise the ability of the partners to keep running the firm and making plenty of money.)