You're conflating two different things. It's obvious in the case of a monopsony, like a company town. Employees can raise productivity all they want, but they will not see a bigger paycheck, because if employees don't have negotiating power, the employer will capture all the cash from that.

Your "pity the poor companies bit" is just bizarre to me. It's true that companies compete for workers, which is why employees have any negotiating power at all. But it doesn't change the fact that employers still have more negotiating power because of relative size. The same is true when companies negotiate with one another: Walmart, for example, is famous for jerking suppliers around because of the relative need.