However, airlines have colluded to gouge passengers for convenience. Passengers are charged more, often much more, for the direct flight. The direct flight has the inherent advantage of the economy of carrying a passenger fewer miles and making fewer stops. Savings from other considerations, such as filling a plane to capacity, must therefore be greater to overcome that. Yet airlines game passengers, seeking to charge more for less. Air fares are notoriously fickle.
One time I was trying to get a trip from D.C. to DFW. The cheapest flight was about $250, and included a stop in Atlanta. The direct flights started around $800. But I found a flight that started in Philly and made its one stop in D.C. on the way to DFW, for about $250. I thought about buying it, and just boarding in D.C. But I didn't as I was sure there was some catch, Yes, indeed. The airline will cancel the rest of your flight if you are not on the first leg. Why would they do that? To stop passengers from dodging their monopolistic convenience levies, of course. I would have been screwed out of my flight, and I think the price of the ticket had I tried it. No refunds for missed flights, you know.
It's a good thing their monopoly is limited. Though D.C. to DFW is about 22 hours by car, vs. 4 hours for a direct flight, if the airlines are obnoxious enough, I'd rather take the car. Instead, I've always been able to find a cheap one stop flight. 12 to 14 hours is about the longest car trip I find practical, and that only barely. Any longer than that and you should really stop somewhere for a night of rest. I very rarely take a plane when the destination is only 4 to 6 hours away by car. With it taking an hour to get to and from the airports at either end, plus another hour to get through security, and the flight itself still takes an hour, the time saved by flying isn't much on such a short trip.