It's hard to argue from a realism standpoint in a video game... but really trying to hit someone in melee range with a projectile from a bow is incredibly difficult, so having a melee option really does make more sense. Hitting things with your bow would largely be more diversionary and defensive rather than a reliable offensive tactic.

Keeping in mind that when I say "Melee range" I mean close enough that your target could be physically hitting you too, not to mention pushing your bow to the side or out of the way .