What Google open sources and doesn't open source is strictly a business decision. The technical details don't matter. Things like Spanner remain proprietary because Google thinks they can make money with it. They charge $10/hour just to have the replicas up and serving traffic; letting Amazon install it and charge $9.95/hour is not something they think is going to make GCP a lot of money, so we don't get the source code. Things like Kubernetes, on the other hand, are open source because Google wasn't "winning" in that area -- to get people to use GCP, they had to get people to break the dependency on proprietary things like CloudFormation. Otherwise, people would just stay at the market leader and not switch to the second place option.
Plenty of people outside Google use protocol buffers, for example; I've run into them at every job I've had since Google, and in plenty of strange places that probably never cross-pollinated with Google (the most surprising place I found them was in Hearthstone). They're pretty popular and people aren't really surprised to see them anymore.
I think there is also a middle ground where people inside Google don't think there's interest, but there is. For example, I very much miss Monarch. I don't think the code is making them a lot of money; my understanding that the Cloud monitoring stuff is completely different. But it is way better than Prometheus or InfluxDB. Queries that are trivial in Monarch you simply can't do with those products. (The one thing I found most valuable in Monarch was that pretty much every query started with an "align" step. And I just haven't seen that anywhere else, so it's hard for me to reason about what the query is actually doing.)
As other people mention, the mere task of picking the transitive closure of dependencies out of google3 is hard. In fact, maintaining a bunch of non-monorepos is a huge chore compared to monorepos once you have the right tool. It's thankless work, literally, so I can believe that's one reason why there aren't more internal Google tools open sourced. But, it can be done if there is some thanks for doing the work. When Google split into Alphabet, work was done to let companies leaving Google take their chunks with them. There just had to be some sort of business reason to justify the tedium.
Plenty of people outside Google use protocol buffers, for example; I've run into them at every job I've had since Google, and in plenty of strange places that probably never cross-pollinated with Google (the most surprising place I found them was in Hearthstone). They're pretty popular and people aren't really surprised to see them anymore.
I think there is also a middle ground where people inside Google don't think there's interest, but there is. For example, I very much miss Monarch. I don't think the code is making them a lot of money; my understanding that the Cloud monitoring stuff is completely different. But it is way better than Prometheus or InfluxDB. Queries that are trivial in Monarch you simply can't do with those products. (The one thing I found most valuable in Monarch was that pretty much every query started with an "align" step. And I just haven't seen that anywhere else, so it's hard for me to reason about what the query is actually doing.)
As other people mention, the mere task of picking the transitive closure of dependencies out of google3 is hard. In fact, maintaining a bunch of non-monorepos is a huge chore compared to monorepos once you have the right tool. It's thankless work, literally, so I can believe that's one reason why there aren't more internal Google tools open sourced. But, it can be done if there is some thanks for doing the work. When Google split into Alphabet, work was done to let companies leaving Google take their chunks with them. There just had to be some sort of business reason to justify the tedium.