Because then you would have to completely redesign everything. Not just the CPU, the socket, the chipset, the OEM reference design, new engineer team to do all this, more months of preparation, aka. All more money for literally no difference. And where does AMD get this money from? Is it worth the cost? The time? The effort and almost 100% guarentee that it won't launch until late this year or even next? When Intel already has an 18 core out? Which means prices will be dropped for AMD which means all that extra money has earned them is actually less.
Business Dice. It makes no logical or common sense to do what you are asking for. You need to remember AMD didn't intend for X399. They simply made a HPC CPU, then realized this could be used for HEDT.