score:5
Airlines sell lots and lots of "fare classes" and they set the price for each of them accordingly. It's partly based on "What the market will bear" and partly what it actually costs the airline to offer them.
As a thought experiment, imagine an airline chooses to have just 3 types of fares:
Imagine a flight with 100 seats. On the day of the flight, 10 people change their plans and don't fly. If they were all "change with no charge" the flight goes out with 10 empty seats and no money to show for it. If they were all "no changes" the flight goes out with 10 empty seats, but they were paid for, so perhaps they don't mind.
Now imagine a flight with 100 seats and on the day of the flight 10 people decide to change to flying that day. Now there aren't enough seats and some people will need to be "bumped" and compensated.
Revenue management or yield management is the job of tweaking these relative prices. If you save "enough" by locking in and being unable to change, the airline will have money it can count on getting. If the "free changes" ticket costs "enough" extra, they won't mind that allowing changes results in wasted seats or costly bumping. But if they make the no-changes seats too cheap, the whole plane will buy them. For example if a no-changes ticket costs $100 and a free-changes ticket costs $500, I can just buy a no-changes ticket, and if I need to change it once I can throw it away and buy another and I'm still ahead; I only spent $200 instead of $500.
You assume the airline can always resell the seat. If that were true, they would not all be in such dire financial straits. And besides, it makes their lives simpler if people stick to their plans. Associating a cost with changing plans tends to make people stick to them.