Why is kbin android app not in F-droid?

Kbin doesn’t have any apps yet. The site is new and apps haven’t been developed yet. There are apps currently in development for it, namely Artemis, but it’s only barely in the alpha stage.

Kbin is open source. Surely the android app should be as well?

There's no correlation between those two.
Just because Kbin is something, doesn't mean the app consuming its API (or well, in this case, scraping, since it doesn't have useful API yet) can be whatever the software author decides it to be.

Having said that, I believe the upcoming Artemis app is going to be open sourced, once the codebase "calms down" a bit. It's still in prototype and I'd assume the dev doesn't want to deal with people reporting issues or forking it at this point.

On my Android phone I use firefox, there is an option called "install app" while in kbin. It is not a real android app, but is close enough. Also, firefox let you use extensions, so install violentmonkey and check here for themes and extra functionality. I personally recommend KUP and collapsible comments.