Exclusive reliance on a perimeter firewall gives you crustacean security: security with a hard outer shell, which when (not if) penetrated offers up a tender and easy-to-pick inside.
Web 2.0 challenges base assumptions of a lot of people, particularly IT professionals working inside big businesses. There's this technology that's just sweeping out there in the consumer space and it doesn't fit in with their world view. They don't understand it - so they resist it.