The reason wages at FAANG are high is because they make a shit tonne of money, and since they've got that much money they can just pay top engineers more than competitors. There's a limit to that - which is how much money are they making.

But if Meta decide they're just going to pay everyone less, what happens? Everyone leaves and goes to Google. What happens if Google decides to pay less? They go off to Microsoft or Amazon. So they can all afford these high salaries - because the company makes a tonne of money, and they have a clear downside to not paying these salaries- they lose top talent.

The cases where other companies are paying less are entirely down to actual hardnosed business, they pay less because they expect to hire less good people and that's a good trade off in a business that isn't reliant on engineering talent. Intel coasted for about a decade not competing for the best talent, why? Because their sales strategy was what dominated the profitability of the company. Eventually they lost their engineering edge and now the company is in the toilet, but that wasn't really a mistake, it was a deliberate choice by management to control costs rather than invest to innovate.