Because if the engineers are mixing genetics around and making them not natural, natural selection won't work because the natural things won't be evolving in the natural way. Say there are green beetles and red beetles. Only the birds like to eat green beetles. Then, some scientists come in, and genetically modify the beetles so that they don't breed green any more. Now, there is an overpopulation of green beetles, and the birds have no food. There is no natural selection any more.