GDS — there’s really only 3 centralized stores of real-time flight/hotel/booking information in the entire world (Sabre/SABRE, Amadeus, and Travelport). Almost every American airline uses Sabre (American Airlines is an interesting case in that it does not
technically in a legal sense, but actually it spun off and sold Sabre in 2000, so a lot of their core systems are forks of each other)
Complexity — Fundamentally you’re looking at a logistics software, except unlike packages you’re dealing with people who aside from expected destinations have travel lengths and time-in-air calculated down to the minute. Also unlike a package, a surprise multi-day trip, unexpected multi-leg journey, one day delay is not something passengers (and crew members) will accept or be at all ok with. And if any one thing goes wrong there’s going to be cascading failures down the line— so much that it may break your company’s entire operating workflow (e.g. Southwest) entirely, and no software can overcome that kind of organizational gap.
Airlines - There’s not many commercial passenger airlines left in the US, especially that fly nationwide. Good luck trying to convince one of these giant behemoths to move to a non-battle-tested system for core operations, especially when decades-old industry software and practices around that software exist.
Entrenched - Sabre is entrenched in airlines around the world. They don’t just provide the booking services, they do the flight tracking, the ticket handling, the upgrading, the in-flight upgrades, missed connection handling, the flight scheduling algorithms, the pricing algorithms, the pilot and flight attendant time tracking, ground crew management, even the terminal software at each gate. To replace SABRE, you would physically need to rip out and then replace software around the world. And because agents don’t work from an office usually, but at the airport, you’re going to need to conduct trainings and provide support around the entire service area, which for the largest airlines is the entire world
Scale — A lot of Sabre’s revenue comes from passengers boarded. It depends on the airline, but I believe the average is that each airline pays 10cents/customer boarded with their software (though with increases in passenger volume each year, it may be less now). Because Sabre is so prevalent, and so many flights use them, they can afford such a price. A company servicing just one regional passenger airline would absolutely not be able to compete on price, at least starting out
Also— Sabre’s software itself is actually reliable! As a corporation it is slow clunky and bureaucratic, but the actual functionality it provides is stable, battle-tested, can handle any travel edge case you can think of, and fast and efficient for those who know how to use it, while also good enough at day to day operations that it doesn’t take too much time to train new agents on how to use it for routine tasks.