Then you’ve never published a plugin to Gradle’s Plugin Portal using their Plugin Publishing Plugin I take it? Because that’s not an open source plugin either.
I could point you to a ton of other examples of other software that you very likely use everyday, that is also not open source. But I’m sure that single example is sufficient to drive home my point. Which is: Regardless of how entitled us devs feel we are to have access to an application’s source code, having access to source code is not necessary.