That said, I can tell you it is a fact that you are looking at several million $ to develop a chip from scratch. Even when using foundry or design house supplied pre-made IP blocks, TSMC will not even talk to you about 16nm chips unless the chip quantities are over 100k/month... Push back to 20-22nm and higher and they are much more approachable but that is also outside our target.
The hashing engine is so small that one could get it developed and samples manufactured for free in the small corner of some bigger project. The mega-dollar capital outlay would be required only in the final stage of mass-production. The semiconductor foundries routinely "waste" much more chip real estate on various testing structures.
So why nobody did that? My limited experience shows that the prospective market is so filled out with liars, bullshit artists, con men, mythomaniacs and associated naïfs (that are honest, but completely indistinguishable for the former classes) that it is completely not worth the effort. The only way to do it is as an intellectual curiosity (in secret) or when from the start you plan on ripping off the fools.
Please don't go blaming the messenger. This is the reality. I'm not going to claim that I know everyone in the Bitcoin milieu. But I've known enough to come up with some generalizations.