PTO regulations are created by the individual States. None require PTO to exist. They do regulate accrual of PTO if it exists, sometimes with unintended consequences for employees.

The origin story is that "discretionary PTO" was created to enable people to take longer vacations than was feasible within the regulatory constraints of accrual-based PTO. It can be abused in other ways but the intent of the people that invented it were employee-friendly.