This would mean that all the necessary drivers for the entire hardware would have to be recreated by reverse engineering. that would take years until you have an even halfway working device.
In the end, you have to poke around randomly in the hardware until at some point a light goes on somewhere. and so on. takes freaking forever. some devices don't have working support after a decade and more of work.
3 devices. They support 3 devices under the "main" branch. And those are not even real devices but emulated on PC.
Every single smartphone they support falls under the "community" branch. that means it was made available by volunteers in their free time. some will never see further updates after getting them to work once.
And the list looks like this:
and so on. there is none newer then 2021 and then it goes very fast down to 2012. see yourself: https://wiki.postmarketos.org/wiki/Devices#Phones
It does run on many phones, but e.g. support for Camera is very very rare because that is one thing you most of the time only get a binary blob for as driver and not the source code. and it is a god damn nightmare to get those things working.