AFAICT Android is a pain to upgrade because of custom closed-source drivers and lack of ABI stability in Linux, so you need to recompile a driver for a newer kernel.
Android is not a Google's internal product, unlike Bazel.
Last week we had a major security vulnerability in Android just because Google can't user the same Linux kernel that everyone else are using.
There exist LTS kernel versions for a reason, you know.
Most Google projects are like this, including Android which is the main reason why vendors struggle with os updates.