Why do we build sand castles?

When we were kids, we built sand castles on the beach.
We knew it will be destroyed. We knew it's somewhat useless but we loved it.

When we are adults, we still do kind of sand castles.
We spend a lot of time "chilling", checking Facebook, watching series, checking Facebook again, watching TV, playing video games.

Why don't we spend our free time being useful to someone/something, helping others, creating meaningful things?