2. Injuries
When other sports practice, they don't have the same risk of career ending injury that pro football players do. When teams practice they have rules for the practice squad so players don't get hurt. Pro football teams don't want their star players to miss out on the regular season were W-L matter. And the players want to play when their stats actually count. When you make these teams play pre-season football you are putting the players health and careers at risk with no reason except the NFL gets money from the TV revenue.