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

Pin weight does matter. The more the better the trailer will tow. That is why 5th wheel trailers handle much better than a pull trailer and if the pin weight is in the 25% area they almost never have sway or any other handling problems provided the TV can handle the pin weight. The only other requirement for a well handling 5th wheel is that it be level. Thus 1/2 tons do not work well with 5th wheel trailers as they can not handle the pin weight that will give a well handling trailer.