If you think about the math, suspending a starting "field" player for five games prevents that player from contributing to his team for the next five games. If a pitcher was only going to be able to contribute in a meaningful way for that one start over that period, a five game rip, even though it really only affects one start, still has the same result of the player not being able to contribute for five games.
Plus, you never know what might happen. If the pitcher is slated to start game 1 and they have to move to a temporarily shortened rotation (Game 3 pitcher gets food poisoning), it could cost him two starts. If they get into a marathon extra inning game, that pitcher is not available to pitch to a single left-handed batter in the 18th inning as the manager perhaps would have wanted.
In any scenario, the same net effect of not being able to contribute to five games holds.
If you had a non-pitching bench player who was a backup and used mostly as a substitute or pinch hitter, how would that be evaluated for suspension for involvement in a bench-clearing brawl? Would they also require a different kind of calculus? Generally, with rules, the simpler they can keep it, the less subjective it is and the less opportunity for controversy over arbitrary treatment of one player vs another.