I think people just say it because it makes everyone perk up and listen. Who knows, though. Even I'm not totally innocent when it comes to describing good weed as hydro, primarily to get my friends to shut up and stop pretending that they are an experienced connoisseur of marijuana and that my supply might be 'beneath' their standards. Words just start to lose meaning so we have to find new ones to replace them. I never call it hydro anymore because, while I do recognize the benefits, I think people tend to overrate hydroponics as a general growing technique. Bad hydro is still bad, good hydro is, well, amazing. But I will always have bias as I grow in soil.