Because ideas happen. Honestly, I highly doubt that if 3D hadn't gotten so popular in the time that the DS has existed, the 3DS wouldn't have happened. If a company just releases one perfect item, no one's gonna want to buy it, say, three years down the road. So they make some tweaks and adjustments and make it better. If companies just released one perfect item, we wouldn't have all these updated iPods, iPhones, and iPads some people so desperately cling to.