They "Exist" to the tune of maybe a couple million per year globally, or maybe ~150k annual for Mac. With the cost of RnD these days, that means, yes, using hardware largely developed for other use cases. That or exorbitant pricing.
If Apple branches from giant monolithic dies to a tile based approach, there would be a bit more flexibility. If making a MacPro that could accept DIMM memory required only an updated I/O die/memory controller, not an entirely new huge SoC, it might happen. But they'll never go that route for the MacPro, only if it helps make iPhones, iPads, headsets and MacBook Airs cheaper.
The last time Apple tried to compete across the majority of PC categories they almost went bankrupt. Apple 25 years ago implemented a '4 quadrant" product philosophy. In 1997-1999 when they were filling those in, one of the quadrants was "Expandable Tower." It was one of the most popular types of computer, and a single motherboard and case design could serve the ~$1500-$5000 market well enough.
Today, the 4 quadrants of computing are Smartphone, Tablet, Wearable, and Laptop. Desktops at all are uncommon and expandable towers uncommon even among desktops. People who think Apple is unique in abandoning the tower haven't tried to buy a PC lately. The market for any tower, let alone a Workstation tower like the 2006-2012 Mac Pro was is tiny.
Personally, if I was leading the Mac team, I'd kill off the 8 PCIe slot Pro and the 0 PCIe slot Studio and release a beefier Mac Mini with front I/O, upgradeable storage and support for the MxMax SoC. Then a "Pro" that supports everything from base M series to ultra SoCs, has 4 x16 PCIe slots, 3 or 4 front "Minibays" (M.2 or custom PCIe x4) for storage or I/O, and 4 3.5" SATA/U.2 bays. I don't think it'd be feasible to add back DIMM slots on this generation, but I'd fight for fitting DDR5 controllers onto later Pro/Max/Ultra dies to use as "SuperSwap," faster and more long lasting than using SSD swap, but a layer above the unified RAM.