As I said earlier, part of the overall management regime is the accountability of the deputy head to deliver the program. For the vast majority of departments, real property or buildings are a means to delivering their programs; they're not core to what they do.
How the department uses those buildings to achieve its program objectives is part of its accountability. If you took that out and put it somewhere else, you then mix the accountabilities. They're going to say “I need those buildings to delivery my program. I'm no longer in control of them.”