Nobody plays Nxd4 because it would lose a tempo
, in so doing bringing White's queen to a commanding post from which she is pretty unassailable. It is almost never good for Black to play N(c6) x N(d4) unprovoked in the opening like this -- not in the Scotch
, not in the Sicilian.
That lemma about "don't bring out your queen in the opening" is a huge stumbling block for so many players. It should be "don't allow your opponent to gain time by attacking your queen with developing moves, or moves that otherwise improve his position", and "don't prematurely determine the position of your queen without reason". Early queen moves are fine, indeed necessary, in many openings.