That has been mentioned quite a few times, but the cost involved in developing forum software that can handle the enormous daily load, number of active members and cross-linked servers involved with Apple's forum is daunting.
Apple has switched forum software twice since I've been a member. They had to, as the former software in use could no longer handle the forum's demands. There are only two or three packages to pick from at this level. Apple went with Jive.
If they took the time and resources to write their own forum software, it would take at least a couple of years to write and test the heck out of it before you would dare replace what the forums are currently running on with your own. It would easily run over a million dollars in development and testing time. Not worth it when you can obtain software already available.
Adobe also uses Jive for the same reason. Sure, they have the talent to write their own forum software, but the expense isn't worth it.