I think there's two parts to this:

1) these historical source code releases really are largely historical interest only. The original programs had constraints of memory and cpu speed that no modern use case does; the set of use cases for any particular task today is very different; what users expect and will tolerate in UI has shifted; available programming languages and tooling today are much better than the pragmatic options of decades past. If you were trying to build a Unix clone today there is no way you would want to start with the historical release of sixth edition. Even xv6 is only "inspired by" it, and gets away with that because of its teaching focus. Similarly if you wanted to build some kind of "streamlined lightweight photoshop-alike" then starting from scratch would be more sensible than starting with somebody else's legacy codebase.

2) In this specific case the licence agreement explicitly forbids basically any kind of "running with it" -- you cannot distribute any derivative work. So it's not surprising that nobody has done that.

I think Doom and similar old games are one of the few counterexamples, where people find value in being able to run the specific artefact on new platforms.