Also, cruise speeds are picked for things in addition to simple range considerations - time of flight is an important consideration, both in terms of sticking to a schedule and in terms of the fact that many maintenance tasks on an aircraft are figured on the number of flight hours. So it can make sense to fly faster, burn more fuel, but rack up fewer hours for maintenance.
There's something called "cost index" which is a simple way to balance the cost of fuel versus the cost of time, and airlines will pick a desirable number which gives them the balance they find most economically effective overall. (There's bound to be multiple threads about cost index around here)