Good question. I don't think people do know that 7+1s are available to those that ask, otherwise they would not buy 5+1s.
I'm reminded of Martin Guitars. Excellent guitars, but they set a minimum price at which their dealers can advertise. Newbies often think that this is the lowest price that the guitar is available. Wrong. The dealer can sell it for any price he wants. Just can't advertise it.
I think shotgun newbies think they can only get 5+1s. That is not correct, you just have to ask.