I have come from a background where a culture of loading up to whatever the machine will get off the ground with was the norm. Indeed, if you didn't you would not have a job for long! It never seemed to be a problem way back then.
On board weighing systems weren't available in those days. It was an eye opening experience when I got to fly with one of these devices. As soon as you commence a turn, take a look at the scale. See what a load it's become! A small percentage overload becomes a big one once you get angles of bank involved.
I'll let the physics experts, and those that sat Principles of Flight exams more recently than I, explain how that works.