Well, Earth ponies are the ponies of productivity. Unicorns have horns and pegasi have wings. Sure, the wings sure have a purpose, they bring the ponies a possible way to get up high places, but unicorns have magic horns, and the magic makes hard tasks really cheap. The Earth ponies can make anything happen without magic, because they can actually do stuff. Anypony with no horn or wings aren't really lazy, and they can still use magic because Equestria is a magical world. Unicrons and Alicorns just turn it into something really easy and that's not really much effort. However don't see this as hate towards Unicorns, I'd be a Unicorn because I'm lazy myself and I don't wanna get up to get things, also I wanna do things impossible in our world. Earth ponies, shortly put, are productive unlike anypony else.