I agree with you absolutely. There's no logical reason for it, unless you accept the mindset that being an adult requires one to comport one's self with a certain amount of dignity. Which is clearly bollocks, because most adults are very far from dignified!

I don't see anything wrong with adults climbing trees or skipping or blowing bubbles in a public park or playing hide-and-seek, or any number of other 'childish' things. If it's still fun, why not?