Shouldn't this have been done years ago? Do all of these manufacturers like reinventing the wheel every time they spin a new silicon? Didn't building custom bootloaders for every device get old a long long time ago?

The systems powered by these ARM processors were typically bespoke embedded software anyway, so not many people cared. Hardware vendor provides a bootloader of sorts and some shitty SDK or RTOS patch, you heap your own custom code on top and ship it. Never update any of the components, that's just asking for extra work.

Now that the processors are powerful enough to replace what would ordinarily run Windows or Linux it's very painful. The Linux code is littered with support files for a bazillion of different boards and hardware platforms.

And even that is targeted at servers I think.