Ah. See, that's a disconnect then. The players are the players, they control the player characters. The DM is the DM, they control everything else. The DM isn't a player in the same sense as the others. They're all playing a game together, yes. But player means a thing. Something that the DM doesn't do.
Well, if something is optional it's not pare of the core game. Flanking is optional, the designers don't assume it'll be used. Likewise, feats and multiclassing are optional, the designers don't assume it'll be used. Race and class are not optional, they're core elements of the game, the designers assume they'll be used.