Open sourcing something like this feels like marketing more than anything else.

They suffer from the same "cold start" problem that social apps do. For the project to thrive, there needs to be an existing fanbase to improve and support it.

Is what makes Wordpress great the fact that it's open source? No. It's the community of builders that surround it.

Would anyone realistically be willing to spin this stack up in their existing company (and sign up for maintaining it)?

I expect you'll get a few git clones from engineers to poke around to see how it was built, out of curiosity, but not much more.

Of course the natural response would be "who cares? They opened the source so you can if you want to!"

That's fair. I guess I just get stuck on the "why"