For an example I’ll use the fairly popular 737-400. According to http://portal.aircraft-info.net/article20.html the maximum seat configuration is 188 seats The empty weight of the aircraft is 76,200 lb and max weight (presumably with people, luggage, and full fuel tanks) is 138,500 lb.
Assuming a plane full of adults at the maximum 188 seat configuration, and an average weight of 210lb per person (170 for the adult human, 40 for luggage), the people weight is 39,480 lbs. Passenger and luggage weight is only about 29% of total loaded plane weight.
Is it really practical to spend all that time checking and weighing in and performing transactions to extract the premium for a few people who are overweight?
they save time and could profile.
It's not just a few people. America is the land of lard a**es.