That's because these are enterprise CPUs really meant for servers. It just so happens that some of them are available via retail channels to use for workstations. The REAL Xeons (E7s) have no retail availability:
http://ark.intel.com/products/family/78584/Intel-Xeon-Processor-E7-v2-Family
The point you are missing is that if the E5-2699 V3s were unlocked they could be both an enterprise CPU AND a flagship ultra high-end enthusiast CPU that people could buy and overclock if they wanted the absolute best that Intel has to offer and were willing to pay the price of admission. It could do both roles with ZERO detriment, compromise or risk to it's original position as an enterprise CPU. Right now, Intel has no ultra high-end overclockable CPU. Don't even mention the i7-5960X...it's not in the same league.