First. You'd have to pay whatever the vig is for each bet slightly minimizing your profits. You're also betting individual teams and not overall wins. You could easily have 17 teams go over because you have a few teams only have one to three wins boosting everyone else's win totals.
For example, last year half of the AFC teams had double digit wins because the Jets, Jaguars, Texans, and Bengals didn't win more than four.