Well, the airlines have made the seats narrower, and the row spacing smaller, to try to pack more paying people into a given flight. But as the people have gotten larger, they don't fit in the seats. What airlines want is a whole bunch of skinny people paying full fare, but that's not what they are getting.
I do think there either ought to be rows set up for larger people (at an increased cost, of course) or some other way to relieve the rest of the passenger list from the discomfort of trying to occupy half of a seat. I sat in a three person row on the way back from the East Coast a couple of years ago, with a couple who were both in the 400 lb range. (They both needed to get seat belt extenders from the stewardess.) He spilled into her seat, which made her spill even MORE into mine; I spent a good portion of the very uncomfortable flight, hanging rather out into the aisleways, getting whapped by beverage carts and scolded by flight attendants. It was not a pleasant experience at all. They were very nice people, but they were simply too big.