That's probably why Apple will get away with keeping iMessage closed (unless the US government pushes them), probably not enough European use to count as a gatekeeper.
You say that like it's a bad thing ("get away"). iMessage has nowhere near a majority and Apple doesn't put in any restrictions against alternative messaging software (…not that they're perfect, and haven't in other areas…).
I don't believe in closed protocols or crappy interoperability. There are several approaches that could improve things, like adopting Google's encryption improvements to RCS so that mixed iPhone/Android conversations are secure ("but that's not in the standard!", well, then, get it or something similar in the standard); they don't have to let others into iMessage necessarily. Apple claims to care strongly about their users' privacy and correctly attacks Google for caring a whole lot less. Encrypted, full-featured messaging would benefit their own users.
Incorrect meme, almost no one is on whatsapp in Hungary for example. We use Messenger, Viber mainly and other social media apps that have a chat feature.
Common case of people seeing that something is common in Western Europe and Latin America and then claiming “the U.S. is the only country that doesn’t do X”. Happens all the time.
So basically you are making my own argument: we use tons of different apps. What would be the selection screen useful for?
Going further: if we download different messengers, it stands to reason we can download different browsers, therefore if safari is the most used it's because it's the one we choose.