I'm just speculating here on why FMS didn't use fuel flow data:
Just suppose that it always use such instantaneous data. Then during takeoff, climb-out, acceleration, and level change climbs, it would show that you can't make it to your destination. Somewhat distracting.
Instead it probably assumes that any abnormalities are temporary, that the gear will be retracted any time now, the climb will stop as per entered profile, etc.
It's not strange, rather logical.
Am I far off? We need a real expert.