“R shach said that you end up learning nach from gemara.”
That might be true if you learn the gemara like Rav Shach. When the gemara brings a passuk for any reason, how many learners today actually know what the passuk is talking about and its relevance to the gemara? How many bother to look up the passuk, its context within the perek, have an understanding of the passuk means and then try and fit it back into the gemara? When a Tanna or Amora (or even a Rishon or Achron) brings a passuk it is because they know what the passuk means already. Its application to the gemara is understood by them. Todays learners?