The thing here is if cross-platform tooling were available, it’d encourage the proliferation of “checkbox ports” which have barely been tested on actual Macs and never optimized for them, which one could argue is more damaging than not having ports at all.
As a game dev I'm OK with having to set up a mac to do initial testing of my port and make sure it works, but I'm not OK with having to do that again every time Apple makes breaking changes to their APIs and threatens to pull my game off the app store unless I port and rebuild. I don't have to do that on Windows or Linux (or on game consoles).
I assume that on Linux you ship with all dependencies included, because if you rely on whatever is supplied by the user’s distro’s package manager provided things will be breaking all the time.
Long-reaching backwards compatibility has never been an Apple thing due to how it encumbers OS development. The longest period old apps were supported was probably in the classic Mac OS era stretching from System 7.5 or so up through OS 9, but that was mainly due to how little the OS changed in that timespan (relative to contemporary OSes and OS X in the the same length of time). With OS X the bits under the hood have always been in a state of flux even if the user facing bits barely changed at all, which makes backwards compatibility extremely challenging short of shipping virtualized old OS versions (which brings its own challenges, such as having to patch 0-days those old versions for however long you plan to keep compatibility for each version around).
Windows’ backwards compatibility is good but has meant that many parts of the OS are effectively frozen in time which has complicated modernization efforts. I suspect that soon MS will bail and start telling people to virtualize software targeting anything older than Windows 8.1 or 10.
The bigger problem I see here is the required shift of paradigm: once users/developers are allowed such things it's no longer "Apple devices own their users", it's a more traditional "Devices belong to their users" which comes with a whole new level of problems that Apple can't possibly deal with.
