Because of all the secrecy in phone hardware.
replies(1):
Lineage and GrapheneOS have proven that reverse engineering is hard. So I'm wondering why can't we have a project that starts from scratch and build the electronics and the OS? In the same spirit of what Apple did with iPhone+iOS. I'm sorry if I sound naive, I just don't realize how hard it is to design a smartphone.
[1] https://news.ycombinator.com/item?id=45586339