It's also a response to skyrocketing work hours. In my field (law), 40 years ago it would have been unthinkable to offer unlimited vacation. Yet, many firms today do just that. But, back then a 1600 billable hour year was a solid effort. Today, 2000 is the minimum requirement. Unlimited vacation is offered because there is no way you can take appreciable amounts of it while meeting the billing expectations.