When the team does the equivalent of locking the door to the cubes and finds that a meeting with the business people the equivalent of an hour of water torture, it comes across and they can't be bothered either, they're busy too. Get and keep them involved, and you'd be amazed at how rarely that occurs.
I find the common perception is that IT causes this, as noted here with your "lock the cubes" analogy. Certainly the classic half-door gateway to the old mainframe shop is a symbol of that. In my experience of being hard to work with, I have found that if I ask the business to be involved it is perceived as if I am asking them to do work for me or making excuses for not just coming up with the solution. Ask them to participate in testing, that sounds like I should have tested better, ask them for more detailed definition, I should know those answers, ask them to consider the financial consequences of asking for more bandwidth, forget about it.