I think it's because people often volunteer for an OT months, if not a year, before the actual release. At that point, nobody can truly know if they are free once it's time to make the OT.
I've never had this problem before because I always make my OT like one day before it's released. In part because I'm a lazy bastard but also because I love making up OTs on the spot. I tend to create some... ehh... non regular OTs :>
That said, I wanted to do some work on my MGS V OT so I gave away every other OT I had volunteered for. One big OT is enough for a year