I'm really sorry that you have this feeling, but this has nothing to do with PhDs. It's impossible to get promoted by bug fixes inside Google, and people are promotion driven.
There were many people who tried hard to fix a lot of bugs, but they usually burn out due to lack of recognition inside Google.
Maybe it's working for them from a bottom-line perspective, but it's made their brand as an engineering company clownish.
I think Google is too big at this point already, but that's an orthogonal question.
Again, though, may not make sense for them from a $$$ perspective. Jank and rough edges galore may be something they're willing to live with.
How this works in my head:
Manager: Could you take a look at the Android developer documentation? Some of it's badly outdated and have you actually tried using it? A lot of the advice is... kinda bad. Also maybe look at a few of these UI components we had the Summer interns make, they've got bizarre implementations and are difficult and inconsistent to customize. Oh and god have you looked at our issue tracker? Has anyone, actually? Like, ever?
Developer: Hey so have you heard of this place called Amazon?
Manager: Uh I mean how about we start a fourth greenfield instant messaging product instead?
Developer: That's better. Amazon? Who's Amazon?
And all of this isn't only motivated by "not doing boring tasks", but also because this is what drives promotions / career advancement. So "launch a feature/project, get rewarded, and switch team" seems like a trend, and it does not encourage to think about long term maintenance / etc.
(It isn't a generality either, but it is a bit of a trend)
Working on improving the quality of the software is not always easy to measure in terms of impact, and hard to get recognition for in the same as shipping a new feature.
Where Google differs, maybe, is that its promotion system is well codified and rewards the "difficulty" of the deliverable. This is skewing the balance to reward adding more complexity in the system for the sake of solving it (you create difficult problems artificially, they aren't intrinsic to the nature of the user needs you should solve).
There are teams whose only role is to work down those tickets, one after the other.
This is why people job-hop for meaningful career advancement. Much easier than dealing w/ the broken promotions processes these companies employ.