Then vendors would get pissed because lots of people would think it's optional / unnecessary and not leave feedback, or they would intend to submit a rating after the product arrives but forget later. Think of all the people who say they will update and never do. The end result is that vendors would be deprived of stats that boost their position on the site. More specifically, vendors who try to protect themselves with FE (which is allowed once you get 35 transactions and $1500 in sales) would be punished. So they would be forced to make a choice between exposing themselves to scammer buyers and losing sales because of reduced ranking, because of fewer ratings. There are trade offs to everything.