This is legally not true. Many countries proscribe not just minimum holiday entitlement but that it is actually taken and in in some cases for a minimum duration. In Germany for example, I was shocked to discover that each employee must take a minimum two week holiday at some point every year out of their total annual entitlement.

That said, nobody says that you have to fly anywhere for your holiday. Indeed if my back was painful to fly with, I wouldn't be wanting to fly anywhere.