I once ran a game where the only races were humans and planetouched, and I think the game lost something because of that. The races of a world help define it as different to our own. We take our own, European-based human behavour for granted until we see that not all peoples are as typically selfish, or warlike.
The elves are unthinkably patient. The dwarves would die rather than give up their traditions. The little halflings will always find a place to fit in wherever they go. Humans can have these aspects, but not the entire human race as a rule, and I think that says something about people today and how little we’ve changed since medieval times. We’re not all brave, or patient, or skilled, and I think that’s why heroes are special, because they always are.
D&D isn’t about realistic player characters - it’s about heroes. Your human fighter isn’t constrained by petty human weaknesses, in part because he’s a hero and in part because the player doesn’t have a reason to. Your hero is always brave because the person controlling him is safe. He is patient because time spent waiting passes swiftly for the player. He always finds his way and takes the difficult road because he knows he will grow from it and is not afraid of the conseqeuences. The fact that he’s being played is what makes him a hero.