Don't agree fully with comments above. Seat belts are also to prevent the driver being thrown into the mast if there is a forward collision with a heavy/ imoveable object.
I have been invovled in investigating a death that resulted in part from a forklift colliding with a building column (hidden by the mast while forklift following a curved route); the very serious injury of a driver traveling at full speed - 22 km/h - with the fork tips on the ground and the tips hit a jump-up at the join between two concrete slabs. And I've heard of other cases though have not been directly invovled so i'm not as certain of the details.

And a further note - the driver of a counterbalance forklift needs to be able to rotate his hips when travelling in reverse - and even withn a simple lap belt and a trucj style tensioner the driver can have problems with the reel brake engaging. A full race style harness would make travel in reverse problematic