Just because someone is 'good' at negotiating higher comp in the interview process, doesn't mean that will have an high impact on the business.

As an EM, a person's current salary is an anchor. If someone is paid at the bottom of the band, but then later the company learns about their high impact, they aren't given a raise, they are told to push harder and get promoted.

And then when they are promoted, they are probably at the bottom of the next band.

if a person threatens to leave, the EM (rarely) can magically wave a wand to increase the salary, but they can promise to put them up for promotion next cycle.