I don't think Apple (or any company) are paying "the most it can pay to all of its employees". Labor is a market, and in a market, buyers don't pay any more than they can get away with. All companies are paying the least they can get away with while still retaining the people they want to retain.

There isn't some fixed bag of money $X that gets distributed to employees, where X is uninfluenced by the market. If the labor market happened to change such that X must be more, companies would have to pay X or do without some employees.