Why don't 5th wheel mfgrs place more heavy items in rear?

The higher the percentage of weight up to 50% the better. 10% travel trailers often use anti sway devices, 20% 5th wheels/goosenecks don't. Neither do 50% semi trucks. Ideally 50% would be the ticket but most people don't own trucks big enough for 50%. 20 - 25% is a compromise that works quite well.

Going back to the OP, the farther aft of the pivot point, (axles) the weight is the better pendulum it becomes. A good reason to put the junk department between the truck and trailer axles.