Another factor is the cost of fuel. If seat width were increased by 20%, six-across seating would require a plane with a bigger fuselage. Bigger fuselage means an airplane that is both heavier. More weight means more fuel required. Also, larger airplane means more aerodynamic drag, which also increases fuel consumption.
So from a practical standpoint, increasing seat spacing means going to five across seating, which would be about a 17% decrease in seating capacity. On routes where planes now routinely fly at near capacity that would mean adding more planes to the route, which is a less efficient way to fly. Meaning higher fares not just because of fewer seats on the plane but because serving the overall demand on that routing would require more planes on the route (or using longer planes on the route). Or the airlines increase fares on the route to bring the demand into balance with the supply.
so, basically, "careful what you ask for ..."