I think (a guess from experience) that a huge amount of Apple work is done under intense time pressure, and prioritized as related to the bullet point list of new features being pushed for that release's marketing communications.
The installer being better isn't going to sell more computers; it ain't really broken, so there would need to be some real incentive to do anything other than incrementally fixing it. (This is not to discount the small amounts of polish they add to these tools bit by bit over time as they are incrementally improved, such as the black full screen background for the system updater.) A total overhaul of such a central system (used regularly by millions of machines during updates) is a gigantic risk, to put it mildly. They also just took one such big risk with the whole APFS migration/system volume thing, which to their credit went well.
Every now and then Apple will actually rewrite an app, like they did to Disk Utility not long ago. This usually makes it worse, and relegates anyone serious to the command line tools (which are fortunately quite complete and comprehensive, even if their man pages aren't).
I have a feeling they spend the most high-quality application developer resources on things like iWork and FCP and stuff that users actually live and breathe every day, not stuff they run a few times a year unattended.
