Well, the process of designing the chip to fabricate. This means paying a lot of electrical engineers to design the chip, probably paying for several iterations

Well, the process of designing the chip to fabricate. This means paying a lot of electrical engineers to design the chip, probably paying for several iterations of lithography masks. Yes, TSMC certainly can make any chip of the desired size - they are already making enough of those - but the development is very expensive. The Mac Pro by itself is certainly not selling in high enough numbers to pay for such a development and considering how long Apple neglected the pro, I am somewhat dubious they would be willing to spend a huge amount of effort on it.