I know plenty of people whose language-learning strategy is basically, "Put myself in situations where I need to use the language and I can puzzle things out. If I get curious, maybe look something up once in a while." It's not quite like childhood acquisition, but neither is it a completely different process.
Sure, eventually you just have to use it. But as an adult learner, you can get going a lot faster by using your knowledge of other languages, understanding of rules, and so on, before jumping in, and it's not always practical for people to just take off and spend months or years in a foreign country in the hopes they'll learn a language while they're there.
My (semi-related) reply here: https://news.ycombinator.com/item?id=9226046
My instructor chose to go full immersion, basically what children are subjected to. The other instructors took the grammar/vocab workbook route.
Only surprise is that education systems around the world continue to waste so many years of people's lives trying to teach them additional languages when they're teenagers, instead of having done the bulk of the work when they were babies, the way that nature's been doing it successfully for tens of thousands (or more) years.