the OEMs care more about their own customization than the core OS, as that's what sets their devices apart. This causes more fragmentation and makes things harder all around. I wish they would submit patches back to AOSP for fixes, and Improvements.
In the end OEMs can do as they see fit to contribute or not .
Also carriers need to not drag their feet approving updates. I think updates should be done via wifi by the OEM and the carrier should have no say, but that's just my opinion.