There's stuff out there, it's just not always easy to find. A lot of the IBM mainframe stuff, for example, is buried in their catalog of "Red Books" found at www.redbooks.ibm.com.
Also search ibm.com for terms like "PL/I", "COBOL", "CICS", "RPG" and so on and you'll likely turn up more of the mainframe stuff. You can, of course, program mainframes in other languages, especially Java. But from an IBM perspective, references to the former batch of terms will generally turn up stuff about their mainframes, or their minicomputer series (mostly what's called iSeries these days, as far as I know. Formerly the AS/400, S/38, etc).
There are also various print books from 3rd party sources that can be found on Amazon. Ex:
The reasons for this are, so far as I can tell, largely what other commenters have already posted. It's just a different culture and way of doing things, and the mainframe world just works a bit differently. And I think a lot of that is because that world is so insular now with IBM being more or less the only mainframe vendor left. I suppose Unisys or Hitachi or Fujitsu or somebody may still make a mainframe here and there, but the market is almost completely dominated by IBM. So the "mainframe way of doing things" is more or less "the IBM way of doing things" and in particular it's a very old skool version of "The IBM way" at that.
Check out:
https://www.ibm.com/z/trials
https://www.redbooks.ibm.com/search?query=%22system%20progra...
https://developer.ibm.com/?q=z%2Fos
Also search ibm.com for terms like "PL/I", "COBOL", "CICS", "RPG" and so on and you'll likely turn up more of the mainframe stuff. You can, of course, program mainframes in other languages, especially Java. But from an IBM perspective, references to the former batch of terms will generally turn up stuff about their mainframes, or their minicomputer series (mostly what's called iSeries these days, as far as I know. Formerly the AS/400, S/38, etc).
There are also various print books from 3rd party sources that can be found on Amazon. Ex:
https://www.amazon.com/IBM-COBOL-Complete-Guide-2020/dp/0655...
https://www.amazon.com/Quick-Start-Training-Application-Deve...
etc.
The reasons for this are, so far as I can tell, largely what other commenters have already posted. It's just a different culture and way of doing things, and the mainframe world just works a bit differently. And I think a lot of that is because that world is so insular now with IBM being more or less the only mainframe vendor left. I suppose Unisys or Hitachi or Fujitsu or somebody may still make a mainframe here and there, but the market is almost completely dominated by IBM. So the "mainframe way of doing things" is more or less "the IBM way of doing things" and in particular it's a very old skool version of "The IBM way" at that.