IBM already has an official Z emulator they could offer for free non-commercial use, if they wanted:

https://www.ibm.com/us-en/marketplace/z-systems-development-...

Starting at $4,780.00 USD* per year

Ouch!

IBM has started to wise up to the necessity of what amounts to "hacker culture" in keeping products relevant, desirable to work on, and secure. I'm not sure what it would take to get that kind of openness where a hobbyist could run z/OS. There are a few ingredients to the success of z.. the hardware, the OSes, and the sheer backing and commitment of IBM. I don't think any of those things are orthogonal to opening up the platform a bit now days. In the past, "plug compatibles" where kind of a drag. If production customers used emulation to run workloads that would cause ecosystem issues. Other than that it is just hard product managment and leadership issue

It is actually obsolete from firmware point of view, and in spite of the efforts reverse engineering it didn’t get right most of the architecture. In the other hand there are other mainframe emulation projects sold by IBM which are capable to scale quite good, but in one hand it was the fear of killing the mainframe (e.g. offering emulated mainframe infrastructure from a cloud). In the other hand it was y he fear of a weak architecture reselling an emulated mainframe without all the warranties that the actual hardware and firmware provides made them sell those only licensed as development kits.

Finally everything is a business decision.

You're kidding, right?

IBM actively shuts down whatever emulation efforts it can...

e.g.

https://www.theregister.co.uk/2008/07/02/ibm_buys_psi/