You might say ‘just fork it’, but I'll see how you do that and keep maintaining a full-featured browser. That would also make it not a monopoly.
Chromium itself has features tied to Google. The ‘Ungoogled Chromium’ project is dedicated to undoing that.
Likewise Android goes where Google wants it to go.
If you check the commit logs or the AUTHORS file, you'll notice a growing number of non-Google authors https://source.chromium.org/chromium/chromium/src/+/master:A...
Microsoft notably has a big presence in the source code and at events like BlinkOn. As each of these folks get their code merged, they gain privs like merge access and start getting tagged as a reviewer in other's change lists
A standardized (and importantly open) platform that executes code exactly the same is the best to execute code on.
Flash was terrific because you, as a developer, didn't need to wonder if it would run the same on every machine. It did.
It was a major hit to my productivity when Flash went away. HTML5 never really replaced it - mostly due to browser compatibility issues.
Flash was not open but chromium is. The future is oddly bright here.
I think you can probably figure out why.