Why do people sit on usernames and never update?

I have been watching an acct for many months, this acct has never been updated. I am looking to acquire that username. There has seriously never been an update on the acct. At what point does twitter jump in and release that username, and lock that acct.
I own a website with that name, and have that name on every other social networking site.
I use it mainly for my website, so why do they let people do this?