Is that specifically for Airbnb? It isn’t the case for Amazon, app stores, TripAdvisor, or Yelp. among other places. Why would Airbnb be different?
Late response, but I think it has to due with social pressure. You may meet the host in person, and feel bad about giving someone you know "personally" a bad rating. Additionally, they are also rating you as a guest, so it may be awkward if you gave a bad rating when they give you a good one, so people skew the ratings up.