It's because the spam bots automatically register and post on the site. Image verification used to work on vbulletin forums but the spam bots have even bypassed that now.
The best method at the moment is a vb mod that asks a question at signup to stop automated spam bot signups:
Im surpised nobody has bothered to upgrade these forums. Seems like they dont care about the forums much which is kindof reflective of everything else DA does. This version of VB is vuln and its only a matter of time since the forum is hacked not to mention spammed up the kerhilk. v2.9.4 is few years old. I guess DA is so busy with updates to the DA software (NOT) they just dont have the time. Sometimes, i think this whole thing is a one or two man show. I'd say, get some more staff and start offering some decent forum help and get your forums software in gear before somebody does a dirty number on it.
Also, as far as updating the forums, I know that vBulletin 3.6.3 had quite a few major template changes. Seeing as though these forums have a custom skin AND are still on 2.2.9, an update to the latest and greatest would involve DA to upgrade to vB 3.6.x financially, as well as alot of theme re-working.