Fuel is not the only cost - all airlines try to minimize the entire trip cost by selecting operational speeds that balances both fuel and time-related costs.

Time related costs are not trivial even if the news only talks about fuel prices...Hourly maint costs, crew time costs, marginal cost of leasing are significant components that are directly related to flight time.

Modern flight management systems use a cost index value that balances the time and fuel related costs - Sperry/Honeywell FMS(0-999) or Smiths (0-99).
The higher the CI value, the higher the various speeds.

Airlines will determine the CI value to be entered by flight crew - either fleetwide, or for a given type, or sometimes for specific routes. The CI will be based on the airline's own mix of time & fuel costs.