If Google spins up a project and then abandons it, how could they possibly be harmed by someone else offering a comparable product? Google has already accepted a total loss on the product, there's really nothing for them to lose here.
It's certainly a win for the rest of us, but how does Google benefit to make it a "win-win", and not just a "win"?
Goodwill and more people willing to try whatever they release next, rather than the current situation of “Oh, Google is releasing a new thing? Pass. They’ll just stop supporting it and I’ll be left in the cold anyway, so no bother even trying”.
Killing so many projects makes fewer people interested in trying whatever they release next, which means fewer users, which means a higher likelihood it’ll be abandoned. It’s a vicious cycle that could be stopped or even reversed if they open-sourced their abandoned stuff.
To be clear, I’m not necessarily advocating Google should do it or that it’s be a clear win with no downsides. Maybe the upside wouldn’t be worth it, but there is an upside.
Next (good) thing they build will probably have greater adoption, due to less fear of "they'll kill this in two years anyway".
When's the last time your (pick your favorite non-technical) relative cared if the product they were trying was open-source?
What non-technical users know is “Google released a project, I invested my time in it, they abandoned it, and I was left hanging. This has happened multiple times so I no longer want to try anything new they release”.
Had the projects been open-sourced, at least some of them would have been picked up by others and continued so non-technical users would know “Google released a project, I invested my time in it, they abandoned it, then someone continued it and I’m still using it to this day. I’m happy to try this new Google thing, because even if they abandon it I won’t be left in the cold”.