It's probably because Pegasi and Unicorns have "better" or more desirable powers then earth ponies do, to be honest I would rather be a Pegasi and be able to fly rather than be an earth pony and be good with plants
My parents don't bother me on the subject, so I don't bring it up, parents don't bring it up, and everyone's happy. My family doesn't really care, but it's awkward conversation since they can't relate to it.