Because many people (although probably not the inhabitants of Flyertalk-land) see the difference in hotels (rooms, location, amenities) but think an economy airline seat is pretty much the same across airlines. That is why airlines are unbundling, so they can show the lowest headline fare, but also differentiate their product with add-ons (extra leg room, checked luggage, pre-selected meals etc), and at the same time earn some ancillary revenues.
And actually there is a trend in hotels to charge for ancillaries too - see the advent of "resort fees", and wifi has often been charged for as well.