Baseball is unique in the importance of the starting pitchers who happen to be selected for any given game. A favorable starting pitching matchup can make an otherwise decisive underdog into a decisive favorite.
In 1987, the Twins got 5 out of 7 World Series starts out of Viola and Blyleven, their two stars. In 1965, the Twins use Grant and Kaat in 6 out of the 7 games. The rest of the starting rotation was largely irrelevant.
That means that the balance and depth that is important over 162 games, is much less important over 7 (or 5 or 3).