Erm, the reason Mac Pro's are still using the Westmere processor family is because the next ones haven't been released yet. The top Mac Pro processor is currently the X5670. The next generation equivalent will be the E5-2670. But it's the next generation. Intel haven't released it yet.
Yet how much do they actually cost when you start putting Xeon and EEC RAM in?
Our work workstations are all basically custom built and still come to around £8k with proper workstation equipment (dual hex core Xeon, EEC RAM, Quattro graphics cards).
Well, there we go then
People buy what they think is best for them.