For publicly traded companies the PTO which has not been taken is actually on the balance sheet as a liability. If that corporation moved to an unlimited vacation time they can make the balance sheets look better. Also, most companies would make people get paid out if they hit the cap. A good amount of people weren't taking vacation and took the extra pay checks.

A big tech company changed the polices when I used to work there for the aforementioned reasons.

In addition, there was also the, "The company is closing down during the week before New Years so that you may spend more time with your family."

Which is a not so clever way of clearing vacation off the books also.

I will no longer work for companies that play similar games.

I urge others to consider doing the same.

I think there's also something to the idea that it was a way to guilt people into work/vacation bleed-over. ("I'll just answer these emails on my day off because they are 'urgent' and because I can always take an extra vacation day later.")

I also got the impression that for some companies it was their founders' means to a loophole for remote work for themselves without instituting an actual remote work policy beneficial to lower staff, because who is going to tell a founder that they are taking too much vacation time, especially when they seem to be working the whole time? Whereas a lower staff member would easily be guilted out of taking anywhere near as much time to work remotely outside of the office. That's certainly something I think about every time I see a company mention unlimited vacations but not a robust remote work policy.