Impracticalities: Deletion of old/unused/neverlogged/etc.. accounts
Why this isn't done and never will be....
Deletion of an account would create various other problems for which there are, with Gaia's make-up, no solutions.
I don't really need to say more than that, do I?
Okay... so why don't you at least change the names on these accounts? (I knew you'd ask that!)
Banned Accounts
It's always a possibility... no matter how unlikely in most instances, that a mistake was made or a decision will be reconsidered and an account will be unbanned.
Zero Posts
Even if an account has zero posts, it doesn't mean that it's not being used. It simply means that account hasn't participated in Gaia's forums. It could be used as a storage mule by someone with a more active account, or perhaps by someone who likes to participate in the non-forum activities on the site.
Users also sometimes make accounts and then don't use them very much... look around, leave, and come back later on when something about the site interests them.
Never Logged In
Many of these accounts are because something glitched and the activation e-mail was never received. Even months after the fact, these accounts are sometimes eventually activated and then used.
Any and all threads asking for accounts to be deleted/renamed/etc.. will be recycled.