Waters down the competition and takes away prestige of making the playoffs. Also, byes sometimes turn out to be a disadvantage.
So why is football the only sport that doesn't include everybody? A basketball team can go 0 - 20 and get in. Have you seen some of the first round district tournament scores?
All sports have distinctive characteristics. Not having everyone make the playoffs is one thing I appreciate about football.
In basketball, volleyball and baseball, teams can play multiple games in the same week, making it more feasible to have all teams in the playoffs. In football, not so.