It used to be more common to have a flexible combination of seats/baggage/fuel. But pilots flip out (or crash) if they can't fill the tanks, every seat and the baggage compartment and come in under gross. So the same airplanes often don't have the "bonus seats" they used to.
Numbers from Wikipedia, from the 1972 PA-32 owners handbook:
3,400 lb gross - 1,788 empty - (4 hours * 15gph * 6 lbs) / 7 passengers = 178 lbs per passenger. The average adult in the 70s was about 160. So you're not going across the country, but you could safely do a day trip with a 90 minute flight each way.
Now the average adult is 180. And they're a little taller than they were in the 70s, but not much. So every passenger has an extra 20 pounds horizontally. So in 2021, you'd be just over gross except that the people can't actually fit in the airplane.
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.
Some planes have more margin: the Cessna 182 for example is a four-seater and can carry full fuel plus 800 lbs, so you really can load it up with four people plus bags and still go somewhere.
But all planes will be close to their operating limits when fully loaded. Even a jetliner will typically be very close to its operating limits on takeoff and pilots have to pay very close attention to this. If you think about it, this has to be the case. If it weren't, the plane would have been over-designed and much more expensive than it has to be, and so it would lose to the competition.
Even with FAA standard people it would be small for 7.
I recall him saying 7, but that was a few decades ago (the plane was destroyed by its new owners in 2006, and I hadn't flown in it since a few years prior to that). :)