after looking at the photos I have determined that 75% of vintage bike people have their seats way too low for efficient pedaling. your leg should be almost straight at the bottom of the pedal stroke.
I'm a mountainbiker, also, and have my mountainbike set up like that.
One thing I have noticed is old bikes cockpits are smaller. Old bikes seats won't go up far enough.
I think people used to be shorter, or just completely smaller, or something.
I have also noticed my feet are too wide for most vintage pedals, and my hands are too wide for vintage grips.
Overall, vintage bikes just seem to be smaller than modern bikes.
Maybe they just didn't have the studies or technology to realize the inefficiency. Maybe people just didn't go out and ride 50 miles for fun or fitness like nowadays.
I know any of my vintage bikes with the seat at a "proper" height would look goofy, and then the handlebars would be too low, anyway.
I only ride my vintage bikes a few miles at a time, so I just deal with getting some different muscles exercised than normal.