Google employees are mostly interested in building new stuff, not fixing broken old stuff.
There's also no central design authority for Google's Android. Each team implements the same thing in multiple different way.
Which is a shame, because there are plenty of people who would happily fix the bugs in the open source version of Android - but Google doesn't accept patches.
This is due to promotion incentives. There's a much higher chance of getting promoted if you make up something new from scratch, stamp your name all over it and then during performance review demonstrate how many users it impacted so the committee promotes you. Often times people then move on to another team right after the promotion, rinse and repeat.
Even though this issue has been acknowledged internally but the above strategy is still the most promising when it comes to moving up the ladder.
How much of that is Google not accepting those patches vs. just the sheer barrier to entry? Building & flashing AOSP on a device isn't the easiest of things.
Uhm, what. They're basically overhauling the entirety of Android all the time: New JIT runtime with M, dismantled old mediaserver framework for security reasons, fixed the notification mess by adding channels, permissions were finally fixed in M, every release is seeing improvements to battery efficiency.
Google employees are mostly interested in building new stuff, not fixing broken old stuff.
There's also no central design authority for Google's Android. Each team implements the same thing in multiple different way.
Which is a shame, because there are plenty of people who would happily fix the bugs in the open source version of Android - but Google doesn't accept patches.