Where I am we do require people to take vacations, usually if they are going to hit their PTO accrual cap (ie, they really have not been taking vacations).

accrued leave is a liability that a company doesn't want too much of.

For people leaving, accrued leave is something that the person could ask to be paid out in cash, or they can take the leave prior to quitting.

As an org grow larger and larger, they'd need to prepare for these to crunch together - and thus, by forcing leave, they can cap the amount of reserves (cash, and people to take over) they need. It's understandable, but of course, frustrates the employee.