Airlines often fly faster to make up for ground delays. Why don't they fly as fast as they can all the time?
It's not true that planes fly themselves; they are flown by computer, and in the not-too-distant future, pilots may be eliminated altogether. [1, 2]
Sadly, with all the advancements in jet technology, one thing that hasn’t changed is the speed we reach our destinations [4].  Airplanes don’t fly faster because they burn more fuel at higher speeds, meaning that it isn’t economical [4]. In addition, operating at higher speeds puts more stress on the engines as well as the airplane fuselage, which causes them to wear down faster [4]. Another aspect to keep in mind is that many airlines operate on paper-thin margins, and therefore try to save on the high cost of fuel [4]. As such, flying faster isn’t in their interest [4]. Generally, the cruising speed of most commercial jet aircraft is somewhere between 400-500 knots [5]. This equals roughly 750-950 kph, or 460-580 mph [5]. The speed of an airplane is most often measured as the speed that the airplane travels through the air [5].

The ground speed, on the other hand, provides a reading of the airplane’s speed relative to the ground [5].