A lot of them do seem to sign up right before they post so who knows. People aren't really making any money on the secondary market anymore so people are trying to get close to face value as possible.
jbird5 said: "When you buy a ticket months in advance things tend to happen to disrupt your plans. Not to mention the financial commitment you’ve made."
Mostly this, plus I guess two-parters double your chances of a scheduling conflict.
some people are just plain undependable (and air headed) and always will be. luckily i found a friend at the last second to go, but i had initially offered my lottery tix to someone who told me was extremely excited about it. i had my doubts, and of course he ended up bailing.