Is it the lack of drivers in upstream? Is it something to do with how ARM devices seemingly can't install Linux the same way x86 machines can (something something device tree)?
On the one hand there is no stable driver ABI because that would restrict the ability for Linux to optimize.
On the other hand vendors (like Orange Pi, Samsung, Qualcomm, etc etc) end up maintaining long running and often outdated custom forks of Linux in an effort to hide their driver sources.
Seems..... broken
I doubt this. Microsoft played a role in standardizing UEFI/ACPI/PCI which allows for a standardized boot process and runtime discovery, letting you have one system image which can discover everything it needs during and after boot. In the non-server ARM world, we need devicetree and u-boot boot scripts in lieu of those standards. But this does not explain why we need vendor kernels.
What is this supposed to mean? There is no device tree to rebuild on x86 platforms yet you can have a custom kernel on x86 platforms. You sometimes need to use kernel forks there too to work with really weird hardware without upstream drivers, there's nothing different about Linux's driver model on x86. It's just that in the x86 world, for the vast, vast majority of situations, pre-built distro kernels built from upstream kernel releases has all the necessary drivers.
ARM devices aren't even really similar to one another. As a weird example, the Raspberry Pi boots from the GPU, which brings up the rest of the hardware.