Apple has complete control over their OS. They can keep their kernel interfaces stable if they want.
Google can't do this with their current development model. They are not in charge of the Linux kernel interfaces. Let's say it's 2019, Android Quaker Oats is here and Google would like to bring it to the Pixel. Quaker Oats is based on Linux 5.0. Qualcomm develops their drivers out of tree and the latest kernel that gets support for the Snapdragon 821 is Linux 4.69. Well, tough luck, Google and Pixel owners, Peanut Butter Jelly Sandwich is all you get.
