Because they want people to consider their OS reliable? If you have software that runs just fine on the previous release of an OS, and breaks on the next one, a

Because they want people to consider their OS reliable? If you have software that runs just fine on the previous release of an OS, and breaks on the next one, are people going to consider the software buggy, or the OS? The perception will largely be "I upgraded to <x> version of the OS and <y> program stopped working" which quickly morphs into "<x> version of the OS is buggy because it breaks <y>, avoid it if you want to be able to do work", and if sustained long enough it then morphs into "<x> OS sucks because they constantly break software". Take away good software and all that's left are Apple's "shinies". If it weren't for the potential game-changer that they have with the M1, I'd almost say that they wanted to kill off the Mac as anything other than an iOS development box.