I don't want to pay extra for that person, and If I know a carrier welcomes fatties with no penalty I will not fly on them, as I consider sitting next to a person who, in effect, occupies not only their seat but 20% of MY seat an affront.
Simply put, we pay for space.
What would it do to mileage or fares to make the planes 14 inches wider. 2 per seat and 2 in the aisle?