Totally agree. An adult working full time on acquiring a language can learn to use a foreign language at an adult level within maybe a year or so. It's just that nobody does that, because there are other things to do...
Children literally need decades to reach an adult level of competence in their native language, despite 24/7 immersion.
Language learning techniques for adults (studying abstract grammar, cramming vocabulary) are very effective and kids don't learn the way.
There is one major disadvantage that adults have though, and that's at acquiring correct pronunciation. I think I've never met a person in my life who didn't learn a language from a very young age and who didn't sound slightly off to a native speaker.
There might be the rare exceptional talent who can do that, but to a first approximation it's not possible as an adult anymore.