Because despite their derivation, words mean what they mean, not what they meant. A long time ago you were right, but the meaning of that word has shifted over periods of time during which Equestria grew to be more accepting. In much the same way, humans here include me in the umbrella of 'people', even though that word strictly means 'humans' -- because there simply is no word that means 'humankind and ponykind' together.