... because the demo isn't the full game, and to do a review on the title you need to complete the game. (not to be confused with First Impression Reviews, which revolve around the initial interaction with the title).
While there are a ton of scammers out there, not everyone is (note to SteamShovelers though - there are a few companies out there that connect content creators/reviewers/etc. with developers - though they do take a little hunting down)