The main means of improving the camera is to increase the size of the image sensor. Which means a bigger camera bump. And you need larger lenses to accommodate the larger sensor, which, again, means a larger camera bump. To get an idea of how far this can go, consider the size of 35mm DSLRs, 35m mirrorless, medium format cameras, large format cameras, ultra large format cameras. As for the iPhone camera getting "better," you'd theoretically be talking about an iPhone camera bump several feet in thickness. Which won't happen, and is why all smartphones use the smallest, lowest-quality imaging sensors available on the market.