[tetromino_]

There are several reasons, but let's start with the most obvious. The reason why you can insert a random liveUSB into a random x86 laptop and expect to successfully boot up a Linux installer is that every x86 laptop on the planet implements the same standard firmware interfaces (UEFI, ACPI, legacy BIOS) which abstract over hardware differences, at least well enough to boot a machine. By contrast, in the Arm world (including phones), there is no universally adopted firmware interface; so almost every phone model requires its own bespoke OS image.

[red-iron-pine]

and more importantly there aren't software vendors imposing demands for abstraction and standardization across all of the hardware vendors a la Microsoft, various unix vendors, etc. Handful of real OEMs, and only a couple of real players in the OS space, all of whom can do what they want their way.