The manufacturer has to release the phone’s kernel source code before any custom ROM development can happen for the phone most of the time for that reason.
There is a reason that GrapheneOS only works on a couple of Pixel phones.
Could be just like windows in that it has every driver for every piece of hardware in the package, just bloating it unnecessarily.
Google specifically designed the Android kernel so that the driver are excluded, unlike the normal Linux or Windows kernel, because, long story short, Qualcomm did not want it to happen.