You're making the assumption that we approve of those lies too. They often cause issues at young ages with trust and conflict. Kids who trust their parents and have been lied to about Santa often conflict with kids who either were never lied to or have since learned better. For some, that feeling of betrayal is a significant memory.

I was never brought up to believe those particular lies, but I do recall kids arguing about it at that age, and I have memories of the way other lies (though perhaps believed by my parents) have caused harm in the past.

I understand some like the whole "magic" of believing thing, but I'm more on the side of just being truthful and letting kids make up their own magical fun.