Because it is simply not enough, it helps a lot, and also puts weight down low which helps when on a slope, but when you tie into a load with your front end loader near its maximum it will still lift the rear without a good ballast on the 3PT. Now with loaded tires and a good heavy box blade, that may be plenty.
Another issue is that the 3pt weight helps to unload the front axle, and the loaded tires do not. But the clearance issues still come up with a boxblade when working in the woods, or near buildings.. Not saying you have to have a ballast box, just saying it is the right thing for some of us. If you live in open meadows, and don't need to works in tight places, then by all means put a "usefull" implement on the back. We all have different circumstances.
James K0UA