You mean this, right?
Since Android is very fragmented you cannot simply install the Android on every phone. You cannot even take the same Android build that other devices with the same chipset use because of the peripherals (camera etc.) and their drivers.
I think that the reason for the fragmentation lies in the roots of Android. It was created by Google as a competitor to the successful iOS and they found that they could only compete if they open-source their OS (interesting classic read with current relevance). Unfortunately they didn’t have higher goals and there is no incentive for phone vendors to contribute their code back and work on a common codebase.
Does this clarify things or do you have a specific question? 
PS.: Splitting this discussion off because it is going off-topic.