As information has gotten easier to send, key decisions have been moved to a central planner.
For example, a flight of mine recently was switched to a different aircraft and delayed by two hours due to a short thunderstorm and its downstream effects.
Once we were ready to board, they informed us that the plane had originally been planning to go to South Africa, and needed significant amounts of fuel removed for safety, which would take two hours… and had not started yet.
This is presumably something that would have been blindingly obvious to the gate agent responsible for that flight, who was sitting there thinking about this flight and this flight only.
Had they been empowered to schedule the resources locally, they could have avoided the extra slowdown.
But it wouldn't have been as blindingly obvious to a central planner, where that one situation was just one random one out of thousands.
It used to be that sending information was hard, so the system had to have good-enough rules of thumb for local empowered decisions.
Another possibility: airlines are trying to optimize their on-time performance, so they'd rather further delay the already-delayed single flight more rather than spread out a bit of new delay across other currently-on-time flights.
Either way, hyper optimization is to blame for lack of resilience for unexpected scheduling changes.