Game Freak Logic
This really is the only answer to be given. Why can't Scyther learn Fly? Why can't Zangoose learn Cut? How can Tauros Surf? It's all Game Freak logic, and there's unfortunately no real answer past that.
Of course, Pokemon are stated to have superhuman powers, so it could be that Pidgey is much stronger than it appears.
Hope I helped!