Why?

It seems to me that if there were as much of a customer base for custom ICs as there is for PCBs, a fabricator like TSMC could easily offer a batch prototyping service on a 28 nm node, where you buy just a small slice of a wafer, provided you keep to some restrictive design and packaging rules.

They already do offer that - it’s called a multi-project wafer or MPW. But it’s prohibitively expensive on a per-chip basis. It’s mostly used for prototyping or concept proving and not for commercial use.

One problem is, you need to create a photolithography mask set for any volume size of fabrication and those aren’t cheap. But that’s far from the _only_ problem with small volume.