5* AND 6* do drop from premium gacha, as you can see many helper teams running double golds or occasionally even a rainbow. Just, their rates are sooooo abysmally low that it takes a real miracle or several thousands of monies' worth flower stones to see them in your pull.

Basically, don't bother whaling till they introduce "exchange big fat wad o cash for the girl of your choosing, no randomness attached" campaign (that made FKG such a hit in DMM) here.