The problem is that phone hardware is incredibly non-standard. Every model requires custom tweaks and regular bug fixes, which is why there aren’t many phones with good Linux support or with enduring LineageOS support or any other specialty OS. Every manufacturer does their own thing and edits Android to fit their hardware, but they generally don’t release the custom drivers or any documentation. The same phone model from a different year or different region might have a different chipset in it. Keeping up with it is basically impossible, by the time an aftermarket developer gets their custom OS build running properly the phone is obsolete.

On the other hand, if a project can pick the hardware platform for themselves then everything is more manageable.