They update just fine, sometimes even on the major versions. E.g. my phone initially had windows phone 8.1 and now it’s windows mobile 10. And despite the phone is 3 years old, I'm still regularly getting OS updates.
That’s why I don’t believe ARM hardware is responsible.
This is why things like SBSA (https://en.wikipedia.org/wiki/Server_Base_System_Architectur...) were needed before ARM servers could even be talked about, and why things like Rasberry PI's are not directly compatible with BeagleBones, etc, at least for the low-level stuff..
Nothing to do with ARM itself; just lack of ARM standards.
This was similar in the early PC days and had similar headaches - compare the various CP/M machines with what became the 'standard' MS-DOS/IBM-PC 'platform'..
Then they mandated secure boot with keys that cannot be controlled by the user, so that other systems can not leverage the platform on W10M hw :p . (That's in the spirit of what BG wanted for ACPI: something somehow incompatible with other OSes -- MS eventually managed to do this with a sidestep, but well, this did not really help their market share :P )
The The PocketPC and Windows Mobile situation was not much different from the Android situation today, i believe.