It's mostly input. Kids have more time. See https://en.wikipedia.org/wiki/Input_hypothesis

I was pretty sure myself that must be the case, until I raised kids in a trilingual household, and it's not even close. Children's natural ability for retention and adaptivity with language is truly mindboggling to witness. A child can overhear a new word once in passing, no explanation but context, and use it in conversation out of nowhere six months later, correctly. It's far from just more time.

In linguistics, this is called the 'poverty of the stimulus' argument, and is used to argue that there must be some kind of soft blueprint for language - whether in the brain (certain areas appear to be highly associated with language - Broca's, Wernicke's, etc) or in logic itself.

"The speed and precision of vocabulary acquisition leaves no real alternative to the conclusion that the child somehow has the concepts available before experience with language and is basically learning labels for concepts that are already a part of his or her conceptual apparatus" -- Noam Chomsky

It's a question that is hotly contended, however.

you probably just lack time

That may indeed be true. Plus exposure - as kid I had that. Now Duolingo and what not…not the same

In my studying (no, acquiring) Japanese I notice a huge difference when I can do 1.5 hours (or more) a day instead of 20-30 minutes. It's almost like the latter is useless. (Edit: The latter is more than enough to learn grammar etc. But that doesn't teach you the language, unlike what they believed in school way back then)