ARM has such a mess. They don't actually make chips. And they don't make a sizable % of the design that goes into chips. There's all kinds of other IP, from power management to USB controllers, all manners of things they don't do.

So then the task falls to the chip makers. Who each are trying to figure out what to do themselves.

These folks don't usually want to do that or have the chops. There's usually one or many different folks taking off the shelf open source and porting it to this particular platform. These folks have a strong strong anti-incentive to do the right thing to upstream support: no one's gonna keep buying sdk's from these software vendors if they upstream support. And it is a pain to upstream support, to spend possibly years figuring out the long term way to do something.

(Notably some good players who focus on mainlining have emerged: Bootlin, Collabora.)

It's all so terrible. Theres some attempts to mature the platform, to make some standards so at least folks can boot something maybe (SystemReady and an array of neighboring acronyms). But man, it's so bad, your question is so searing, so obvious. This whole world systematically seems unable to do the right obvious good thing for itself, has resolutely remained a shitty backwater for 3+ decades versus x86.