Because in Google's conceptualization, organization is horizontal and extremely employee-driven.

It's ultimately the individual employee that knows whether or not a meeting is relevant to them. And they can only know that if they have information on the meeting.

So someone can just up and invite themselves to a meeting I set up and did not invite them to?

Sounds like an entirely toxic environment.

Not really. The principle in play is "We're all adults; don't be a dick."

Proper etiquette would be to ping the meeting organizer and ask if they mind if you sit in. If they don't, no problem. If they do, you work something out relevant to the reason you wanted to sit in. But one can't even start that conversation without knowledge of relevant meeting topics.

The fact I know what meetings my coworkers are having doesn't imply I should be crashing them any more than the fact I can see when my coworkers are at their desks implies I should be continuously bothering them.

Because that’s the domain-wide setting that Google configured for all of its employees accounts.