Low volume simply means higher unit prices, doesn't it? The Mac Pro as an explicitly professional high-end machine is OK to be expensive. Its USP compared to fast x86 machines is that it runs macOS.
That is until you run the calculation when the cost of doing it are in the tens of thousands per unit. Suddenly it doesn't make much sense.
Sure, Threadripper makes economical sense for AMD only when "subsidised" by EPYC/Ryzen sales and the shared chiplet architecture. But Apple has deep enough pockets for halo products. They are allegedly repurposing some of the binned/sub-par silicon into internal server hardware anyway.