Airlines have a pricing strategy known as "yield management" or "revenue management" - they charge less for some seats than others, and expect these seats to be bought a long time in advance. They know that only a certain percentage of their customers are able to buy seats well in advance, and that those customers wouldn't fly if they couldn't get inexpensive seats.
A speculator could buy a $100 ticket and then offer it on eBay close to the flight date for $200. If more than half the seats this speculator bought were sold this way, the speculator would be making money. But the airline, which wants to sell seats close to the flight date for $500, would not. In fact very quickly the speculator enjoying selling 75% of tickets for $200 would see it fall to 0% because of another speculator selling them for $150, and then later another for $110 and so on. This is just how reselling markets tend to work.
By insisting that a ticket is not a commodity to be bought, traded, resold, and passed around from hand to hand, the airline is able to keep its complicated pricing structure in place. Overall, this is a good thing, because those last-minute high-price tickets cover a LOT of the cost of the flight - their existence is what keeps the long-advance-notice tickets so cheap!