They do, by not giving appropriate raises. Adjusting down nominally is typically done via terminating everyone and telling them to reapplying.
Though, in at least a somewhat healthy company, adjusting down nominally leaves too much of a bad taste, so layoffs are used instead where the remaining employees’ pay to qualify of life at work ratio goes down (more work responsibilities, more hours, less amenities at work, etc).
This is where pay transparency [1] comes in. If employees can see what other people are making and it's clear that old hands are getting the shaft, then old hands will leave.
Also the last few places I've been at have made a conscious attempt to reset once a year or two to combat this effect. I can't speak to prior employers as that was before I started to be privy to how the sausage was made. Companies that don't reset compensation aren't worth working for.
Transparency helps a lot, but is not sufficient. Labor buyers can also be knowledgeable about their labor supplier's other options, so if they are the only game in town (such as in smaller towns/metros), they might choose to adopt a more take it or leave it attitude.