I believe it's mostly about their learning methods. Kids learn faster by playing and exploring, so they need to do things to understand them. Adults, on the other hand, use their previous knowledge and experiences, which allows them to think about the skills they need to learn differently than kids.

Certainly, adults need more time to learn new things. Children, when learning a new skill, just see it as it is. Adults, however, often question what they learn, especially if it contradicts what they already believe. This makes it harder for them to learn new skills quickly.