I actually do do this. I'm not sure who the 'overwhelming majority' is and where you got the data (link would be appreciated) but everybody that I know that runs these is doing so on their own infra.
The only open models that are "almost as good or better in some cases" require massive amounts of RAM. I posit that most people cannot afford a decked out Mac Studio, and therefore run the smaller "flash" variants on more normal devices. The issue is that those are nowhere near frontier-level in terms of capability.
Not just your local machines.
Enterprises are where you see this adoption. Legal, finance, tax; sensitive context where the data must be contractually opaque to external parties.
PCIe is incredibly powerful tech.
Managed infra is, by its very definition, not your own infra. It's infrastructure someone else sets up and manages for you.
Same way you say "my apartment" and not "my landlord's apartment". It's your place while you're renting it.