"They do not have milk."
Translation:У них нет молока.
My PONS cheatsheet says the Russian verb бьить (meaning "to be", not "to be located") vanished almost completely in its PRESENT tense. - only the PAST and FUTURE tenses remained more or less completely - in the PRESENT tense, the only remnant is the 3rd person singular есть, meaning "there is". That is used in the у + possessive pronoun + genitive construction to mean "have".
I guess all the other PRESENT forms just weren't necessary to express what people wanted. So the real question isn't "why does нет not have есть", but why did only есть survive; what was it needed for? To express possession. If one word suffices to exoress lack of possession (нет), why use two?!