I would hazard a guess that the cost of any real increase past that "standard" must be... in the view of the airlines/aircraft manufacturers.. uneconomic...
Put it this way.. the last warship I served on was capable of > 30 knots..
..and was powered by 2 LM2500 gas turbines (the marine version of - you guessed it - a jet engine designed for aircraft)
We used ONE of those engines for any speed up to 25 knots IIRC.. the second engine was needed for the relatively small increase to "top" speed...
I suspect aircraft are the same deal.. massive increase in power required past a certain point for relatively modest gain....