Remember you're not just doing the same weight over and over, nor can you make big jumps. So if you're training with more "real" objects like sandbags or stones, it's a problem. Say I lift stones. Today I lift a 100lb stone for reps across. Next workout I should try to lift 105lb. Do I have a 105lb stone? What about 110lb, 115lb, etc? There's also the issue of size. How big is a 400lb sandbag? I might have the strength to pull that weight off the floor, but if it's too cumbersome it will be a poor training instrument. For example, you might have a 500lb deadlift, but won't be able to lift a couch all by yourself that only weighs 100lb because of the weight distribution and sheer size. It's just not practical after a certain point.
Also, how do you get the dumbbells there, where do you get 150lb+ dumbbells, and how do you increase the weight by only 5lb total?