Why updating Android without vendor help is a nightmare
The operating system runs on billions of smartphones and tablets, made by all sorts of companies. It primarily targets the ARM platform. A count can only be kept when a device is activated and many users choose not to do so.
Android is based on the Linux kernel which is released under the GNU General Public Licence version 2 and is free software; modifications can be made but if the modified binary kernel is distributed then the source needs to be made available too. All other components are released under the Apache licence, which means that there is no obligation to divulge any changes; in short, these can be locked away.