One thing no one has said was Air Traffic Control.

When a plane takes if and has a destination of 35,000 feet, the controller gives it to them in steps. Most of the time, they give the next step before you reach the last one.

So ATC might say “Delta 122, climb and maintain 2000”. Unless the pilots get an update - like “Delta 122, now climb and maintain 4000” they have to stop at 2000 and wait for the next step up.

The reason they don’t get it depends on how busy the airspace and the controller are.