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.

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.