But they do.
Every company, movie, project, ect ... has a budget. So my company COULD pay me $30 million dollars a year, but to get under budget and not go bankrupt how many jobs would that cost? So instead of "peer salary cap" my company uses "Comp Ratio". I am at the very low end for my title right now, but with any luck, I my comp ratio will improve by the end of the year. And then for some, they can get RSU, discretionary bonuses, or extra PTO as bonuses for exceptional work, which is also budgeted as well.
So while virtually no company calls it a "Salary Cap" they do refer to it as "headcount", "payroll", "comp ratio" or any other various group of words to describe how much of the total revenue can be used to pay employees their salaries.