Sometimes I arrive at the airport early enough to catch the previous flight to my destination on the same airline. But even if there are empty seats on that flight and I have no checked bags, the airline wants to charge me a change fee plus the difference in fare based on the fare code of the seat that's available. That sometimes amounts to doubling the ticket price, so I prefer to just wait two hours in the airport for the flight on which I'm ticketed.

But I'm trying to figure out why it's to the airline's advantage to make this change cost prohibitive. Flying with that seat empty seems like a wasted profit opportunity for them, because letting me take it would open up a seat that could be sold on the flight where I was originally ticketed. Plus, it makes a happy customer who is able to arrive at his destination a bit earlier.

Even if they charged a small change fee ($35-50) and imposed restrictions (no checked bags, option only available within 30 minutes of boarding, can be bumped by any other passenger with a higher fare code trying to do the same thing), it still seems like it would be to their advantage to accommodate me. And if implemented company-wide, a cascade effect could take place, where the latest flights each day would have the most available seats that the airline could resell at a high price to last-minute travelers.

From what I can tell, everybody wins. What am I missing here?