I think it's because children's brains are more flexible. They can learn things like languages or new skills just by being around them. Their brains can make new connections quickly, so it's easier for them to remember and understand new things. Adults' brains are not as quick at picking up new information.

Another reason is that children have fewer things to worry about. They don't have jobs or families to take care of, so they have more free time to learn and practice new things. Adults are usually too busy, so they can't spend as much time learning something new.

Also, kids aren't afraid to make mistakes. They just try things without worrying about failing or looking silly. Adults, on the other hand, often feel embarrassed if they make a mistake, so they might not even try. This makes it harder for adults to learn new things.