Why can Water type Gyarados learn Thunder? or Slowbro learn Flamethrower? How does Jolteon learn Shadow Ball? Why does using a strange machine suddenly allow a muscular Fighting type like Machamp to suddenly shoot Fire from its mouth? and why can't every pokemon with wings learn Fly? why can't Gliscor but Rayquaza can?
if you spend too much time analyzing it you'll drive yourself mad. just accept it for what it is: Game Freak making the game balanced in terms of movepools. if they had to make specific moves for all 600+ pokemon, all based on that pokemons physical appearance, it would be WAAYYYYY too complex.