Just to buttress and embroider around your point that a fab is not a small business:
If there was a realistic way even to go from bare wafers to non-trivial custom chips in a small-batch fashion, you can bet there would be a cottage industry around it. I would love to live in a world where I could manufacture custom silicon as easily as I can manufacture a custom PCB or custom mechanical part.
But as it stands, quick-turn, rapid-proto "micro" fabs are obscenely expensive, to the extent that if you aren't absolutely certain you need the performance gains from custom silicon, justified by years of R&D that confirms the inadequacy of a multi-chip solution, then the idea is killed before any layout engineer is contacted.
Microfabs are either operated by research institutes, or they're booked solid for years, and basically printing money.
The closest thing to that cottage industry is IMEC.
IMEC is a lab, not a fab. They have partnerships with all major fabs for driving research forwards and making prototypes and concepts, but they don't manufacture anything there, it's still up to Samsung, Intel or TSMC to try out whatever IMEC comes up with.
They may have lasers, electron microscopes, probes, etc on-site for testing what Intel or TSMC ship them and verify research results, but that's pretty far away from a "cottage industry".
Intel and Samsung are the true "cottage industries" as they do full vertical integration of IP, R&D and manufacturing under the same roof.
IMEC is more like the UN of semi companies, a place for them to come together, share knowledge and results and talk standardisation based on that.