Because the newbies will work for less. They will work longer hours (to learn the ropes.) They don't have families and health issues, so they are cheaper to employ as well. They are less aware of their rights and have less long term outlook, which would steer them away from these sorts of HR miasmas.

Thus they are just the filler needed by BigCo to compensate from the continuous braindrain. Once they have the skills and know the ropes, they'll either get promoted or leave for an employer of choice.

That may be the case, but it doesn't mean the above statement isn't just factually wrong.