[pale-hands]

Good question. Maybe (just speculating here) because it would be dangerous to land with too much fuel on board? Planes dump fuel for that reason when aborting shortly after takeoff, but that may only be a problem with tanks close to full.

[xvf22]

After a transatlantic flight it's highly unlikely you'll land over your maximum landing weight. That situation usually happens only when they have to land within an hour of takeoff.