Because it wouldn't accomplish anything? You could say that about anything. Make a weapon better, and then make it a power weapon, and then... well, then it's a power weapon.
The DMR would be really boring as a power weapon, even at 3SK. It'd just be a less powerful sniper rifle. What would the point be? Might as well remove it at that point. --- Oh boy! I can't wait to bomb some dodongos!