Here's a Piper ad from 1966 with seven adults and their bags: https://i.imgur.com/fWyArRH.png
"The SIX will carry up to seven adults and their luggage - in real comfort"
And here's an ad from 1969 with six adults in it (and some rifles. a different time!) https://i.imgur.com/kGMrENf.png
I can't read into the minds of the engineers who made the airplane. Maybe it's true "I don't think they even imagined 6 adults as a typical cabin load". But we can see that the marketing department at least tried to make people think it was.