It also has to do with the wide diversity of languages spoken. The elementary school where my kids go put out a statement during the pandemic that there are 32 different languages spoken by kids at home. They had gotten many requests for school communications in more than just English and Spanish, and had to explain why that wasn’t feasible.
So there are a ton of bilingual kids in their school, but my kids could learn the 4 additional languages spoken by the kids in their classroom, and the following year they would need to learn 4 entirely new languages. They learned to count to ten in several languages, but like you said, they will never have the opportunity to become fluent if they don’t go somewhere less heterogenous.