Imagination...

Why is it that humans can't live out any fantasy they come up with in their imaginations? I'm not talking about having sex with 10 girls or becoming a rock star, I'm talking about things that are "truly" unattainable, like flying or having sex with 10 mermaids.

Can anyone explain? (no)
Maybe after we die we live in a world where everything is controlled by our imagination. Probabaly not though.