Airlines have fare inventories in which they will have a certain number of fares available per flight.
Yes, full fare and first class tickets are great revenue for airlines, however they are associated with very good benefits too. For example, preferred seating which is blocked until just before the flight, some entice with first upgrades, and best of all- freedom. Tickets are fully refundable and have greater flexibility to reroute or change times and dates.

These in turn cost the airline to hold on to these seats that you might otherwise refund on. Also, in the world of sales, if you can create a sense of urgency such as a sale on a 21 day advance ticket, you are in turn "guaranteeing" the airline revenue for that flight. These in turn cost the airline money because you are discounting the price and possibly diluting the product. This is why there is so much confusion over fare structures and the like. In the end, it is all about supply and demand. The airlines will charge the prices they do as long as people are willing to pay for them. Yes, it may hurt business but there are also some safeguards for individual passengers such as emergency situations and bereavement fares.

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