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While it's a shame us Anglophones are mostly monolingual, there is a clear reason and pattern as to who is trilingual. If you're first language isn't English, you will always benefit from learning it. There is not an instance, except for maybe being born in North Korea, that it couldn't help. But that's the default language every non-English native speaker would do. If there is another lingua franca in your region that's not your native language, it is likely you learn that before English, but if you learn another, you learn English. But once all of that is done, in this industry, Japanese is important and can help you get into Hololive.
If English was your first language, there is not another language of such global importance to learn, so that reason is gone. If it's your first language, there is likely not another local lingua franca you would need to learn since English would already be that, unless you move to another region. If you have no experience learning another language in a real way, then learning Japanese is a major fucking hurdle. However, it can be done. But if you had good reason to learn Japanese as an English speaker, learning a 3rd language would serve no purpose. So the most you would ever see is bilingual, if even that.
As for Japanese native speakers, they live in an ethnostate with little migration out and plenty of entertainment. And their English education is pretty shitty, so monolingualism is pretty common, even if learning English would be beneficial.