Consider the typical board states you get into with your creatures when playing mono red or mono green. In which case would a one mana +3/+3 instant be more useful? I feel red wants Brute Force more than green wants Giant Growth.
Interestingly enough, "the one green doesn't use because its [sic] terrible" has something in common with bounce spells: It and "the one blue doesn't use because its [sic] terrible" are probably the first of what Maro now calls lenticular design.
Red pumps combo with Feather, the Redeemed and Dreadhorde Butcher. Green lacks as many cards that give your instants extra effects like Tenth District Legionnaire or Burning Prophet. Giant Growth doesn't play into complex strategies to the same extent. There's a reason you don't see it much.