Consider an example where we have a business traveler "Bob" and a leisure traveler "Larry". Bob needs to get to LAX tomorrow to put out a fire at a client site. Larry has a trip booked to LAX tomorrow, but can't go because he's sick. Larry has paid $500 for the trip 3 weeks ago.
Today: Larry cancels his trip, and maybe, if he's lucky, gets an airline credit for the original price of the trip that expires in a year and which may be hard to use for his next trip. When he cancels, a seat opens up on the plane, and the airline sells it to Bob for $1200.
If resale was permitted: Larry auctions off his ticket at an airline ticket reseller. He gets $700 from Bob. So if resale was permitted, Bob's business saves $500, and Larry makes $200, and the airline looses $1200-$1700. You can see why they hate resale.