I haven't touched a mainframe in over a decade(and i was only ever doing user administration on one) but from my experience with hpux and other legacy platforms a big part of the reason is that the actual information about the core platform lives in old hardcopy textbooks and paywalled vendor documentation, and because a lot of those platforms are "feature frozen" the old textbooks are still valid enough.
And on the application side you are typically dealing with databases and Runtimes(Java mostly) that also exist on Linux and have an large Linux centric community where any legacy programmers and admins kind of drown in the masses.
There is a whole bunch of companies out there with large COBOL and ALGOL code-bases but but as the problem here is less about generic mainframe skills and more about knowing how to work large COBOL or ALGOL projects.
And on the application side you are typically dealing with databases and Runtimes(Java mostly) that also exist on Linux and have an large Linux centric community where any legacy programmers and admins kind of drown in the masses.
There is a whole bunch of companies out there with large COBOL and ALGOL code-bases but but as the problem here is less about generic mainframe skills and more about knowing how to work large COBOL or ALGOL projects.