Every place I have worked has capped the amount of paid time off employees are allowed to roll over to a new calendar year. In many cases it's a "use it or lose it" scenario, where accrued time is lost if the employee is over the cap.
One year, the company I worked for significantly reduced the amount of rollover allowed, in order to "prevent employees from disappearing for large amounts of time while we have important projects running", but this had the exact opposite effect. My project lead literally disappeared for 5 and a half months so he would not lose any of his accrued time.
The next thought is future liabilities. Maybe they are worried about having cash to pay someone out when they leave the company or retire, but, simple accounting should be able to manage that by pre-paying into a fund each year and only pulling money out when someone uses paid time off. The idea being if they had the cash to pay it on 31 December, then they still have the cash to pay it on 01 January. It could even be an interest-bearing account to avoid inflation issues. This makes the "we may not have the cash available if you store up too much paid time off" argument rather thin.
The next thing I can think of is maybe they are truly looking for a good work/life balance and want to make sure their employees get time off. That's kind and all, but if there are employees who would rather work a lot now and take a lump sum when they retire, I think that's a valid scenario for employees who really thrive on that sort of thing.
Tax reasons possibly?
This leaves the last thing I can think of: maybe this is just the way it's always been, and no one has really thought to question it. But that seems rather simplistic.
So to get to the question, why would employers in the USA cap paid time off accrual?
Answers such as "that's the HR policy" are not super helpful, as I'm trying to get to why such a policy exists.