It is much easier as a Google employee to publish open source projects under Google's copyright[0] rather than getting them to grant the copyright to you.
To have Google give you (the Google employee) full copyright of projects you work on while at Google, you need to go through a committee[1] that reviews the project to make sure it doesn't collide with some other project Google already is working on. As this is really hard to do for many projects, it's easier to just let Google keep copyright ownership of it and have it opensource under them.
I can't really see an argument to be had that it's not Google's property if it was worked while on the clock.
Considering how protective most companies are of IP, I'm shocked Google even has a mechanism by which a person can take back things they've worked on in office at Google.
To have Google give you (the Google employee) full copyright of projects you work on while at Google, you need to go through a committee[1] that reviews the project to make sure it doesn't collide with some other project Google already is working on. As this is really hard to do for many projects, it's easier to just let Google keep copyright ownership of it and have it opensource under them.
[0] https://opensource.google.com/docs/releasing/
[1] https://opensource.google.com/docs/iarc/