Airlines are trying harder than ever to take off with exactly the right amount of fuel (required fuel for the flight plus FAA required reserves). Every extra gallon of fuel on board makes the plane heavier and therefore it burns more fuel to carry that excess fuel. That adds to the costs and they are doing everything to keep costs down. However, having the airplane on the ground is also costing money, since the flight is going to be late, meaning they have to keep ground personnel around (possibly with overtime). That fact may be too complex for people in charge of just making sure that the plane takes off with the right amount of fuel.

I remember one time in the late 1990's on United, getting ready to take off from LAS, when the pilot came on and said "we have just a bit too much fuel on board, we are going to pull over here and burn a little bit off before we take off." Sure enough, we taxied out over to a pad off the B taxiway approaching 25R, he set the engines to about 30% (my guess) and we burned fuel for about 5-10 minutes, then we were good to go. I doubt I will experience that again!