Lets say a worker has nominally 10 days off per year. What the company does practically is if you take more than 6 days off is start writing you up. Theoretically you could take 10 days but any infraction would result in termination. The practice ensures employees are always dismissible for cause unless they don’t use vacation days.
Not all companies use such tactics on their workforce, but enough do to make it a systematic problem.
Mandatory payout at 100% for unused PTO, and a ban on use-or-lose would be a good start.