There are people willing to pay up to $100,000 or even more for an HP Z8 G5 with multiple high end GPUs and tons of RAM and storage. So there is a market for that.
...but that doesn't mean that there's a market for a
Mac with those specs. Those "multiple high-end GPUs" (probably NVIDIA or AMD) aren't going to magically get faster because they're slotted into a Mac rather than a HP. In fact, they're gonna get
slower because (even if Apple added PCIe GPU support) a Mx Ultra only has enough PCIe lanes to drive a single 16x GPU. Most must-have software at that level is available for Windows and/or Linux, the "user friendly" Mac UI is no big deal if you're immersed in hugely technical pro software, and the power efficiency of Apple Silicon is drowned out by those big, sweaty GPUs.
The reality is that Apple Silicon is
great for the tablets, laptops and small-form-factor systems
that account for the vast majority of Mac sales but if you want a multi-PCIe-GPU workstation, a M-series chip
just isn't the tool for that job. Apple doesn't have anything special to bring to the "big box'o'slots" market - and they don't need to.
Apple could easily offer a Mac Pro with four M5 Ultra for example.
Of course you can build a cluster of four Mac Studios, but some people want it in a single case.
The only advantage of that would be if it used "raw" PCIe for RDMA interconnects to get an edge over Thunderbolt. An Mx Ultra still only has 16-24 lanes of PCIe to play with, but that's usually a generation ahead of Thunderbolt. Not sure how much that would improve performance, and they'd need to design a 5xM5 Ultra logic board with all the PCIe plumbing, and a case... to serve a tiny market.
What they have with the Studio is something that performs exceptionally well
in relation to its size and power consumption - even if it can't compete with a rack stuffed with HP's finest - with a (relatively) large potential market, with a
software-only (well, plus a bunch of Thunderbolt cables) route to scaling it to multiple machines for more specialist apps. 4 Studios in a stack is still smaller and more power-frugal than a massive tower - and there's plenty of scope for a third-party to come up with a better way of organising it than a generic mini-rack designed for
Raspberry Pi clusters (as shown in the video) and regular TB5 cables.