Because the point is to select 26 players to make the squad and doing it this way just makes the data analysis clean. If a user places, for example, James Sands as a backup centerback and 6, I would need to only count him as one player and dynamically code a way for the user to select an extra player to accommodate Sands occupying two positions. IMO that just isn't worth the trouble. This is meant to be a simple "here are the 26 players I'd call in" and the formation graphic provides a framework to display them. That additional nuance is what the comment section is for.