Do remember that this 32-core Threadripper offers quad-channel memory, 64x PCIe lanes, and ECC support (!!). So the Threadripper offers pretty much everything a professional wants.
The only downside to Threadripper is that its multi-threaded performance is similar to a quad-socket / quad-CPU design (~200ns to ~300ns latencies to memory, depending on how many "hops" and NUMA nodes and stuff...). So audio professionals and gamers who are IIRC latency bound may want to stick to Intel.
But almost everyone else is bandwidth bound (video editing, 3d rendering, compilers, web servers, LT Spice...). So most professionals would probably prefer a Threadripper.
It's the case that building a comparable computer (in terms of the specific iMac Pro hardware) is very similarly priced BUT requires labor, the purchase of an OS and additional peripherals: https://youtu.be/SONKIJd8xRM?t=187
That said, your point that Apple could've chosen better hardware at its price, is reasonable but off topic.
[update]Sorry he said $20,000. SO you are right
Note that most price comparisons show that the iMac Pro is actually quite the deal.