The fares are set to try to fill the planes, not cream the passengers.
Hmmm... I'm not so sure. There is absolutely no point in filling an aircraft by selling cheap fares if its making no money, there is considerably more value in having an emptier aircraft where the pax pay much higher fares, and turn a profit.
Obviously the extent to which this occurs depends greatly on which route you're talking about (sector length, airport charges etc) and the type of passengers that use it. Business routes will
generally have higher fares, operate with more empty seats, yet make decent money.
I do realise it is far more complex than this, but the sole purpose of an airline is not simply to put as many bums on seats as you can, but to make as much money as possible.