Sometimes it's not ignorance, but a greater selection of options through Orbitz/Travelocity, etc ,which cause people to use them instead of the airline site.
Last year we booked from Orbitz because it offered the exact times we wanted. They did this by booking us on two different airlines and using two neighboring airports. It was the same average price as the airline's own site, but we were able to get more specificity through Orbitz.
This year, we booked right from Delta's site, because date, time, and price were right.