It's probably more of a node thing than a fab thing. You would have a much easier time getting the fab to do random stuff for you on a legacy node compared to a leading edge node.

Leading edge nodes are basically black magic and are right on the edge of working vs producing broken chips.

You as a customer would never want to be in a position where you are solely responsible for yields.

There are only a few Fabs with nodes smaller than 28nm. Yes, all fabs are like that, with exception of a few experimental tiny labs at research institutes or universities.

There are open-source PDKs[0] for education, research, and R&D - but they extremely rare and decades behind state-of-the-art.

Nobody wants to give away trade secrets, so everything remains proprietary and behind an NDA until it has become completely obsolete.

[0]: https://www.skywatertechnology.com/sky130-open-source-pdk/