It's worth noting also that people always underestimate how much active work a child goes through in learning a language. They are under constant tutelage for years by family members who correct their pronunciation and language usage. They go to school where they have to dedicate hours on spelling, grammar, etc. And they have to use the language constantly in service of learning other things. Proficiency doesn't just appear from exposure.

As an adult learning a second language you generally won't have anywhere close to the same level of language exposure or learning support.