Cellphone cameras are a tradeoff due to the limited space you have to put a sensor. Something capturing 0.3MP (480p) is going to do a better job in low light than something capturing 2.0MP (1080p). To get an HD sensor to perform better in low light, you need to make it physically larger. This way, it has more surface area to capture light. This would make the device larger and the camera more fragile. So the tradeoff route is taken, and you get a camera that is "good enough" in most situations, and in some cases even "really good" if conditions are ideal.
High megapixel numbers sure make for good marketing, though; since most consumers would expect an 8.0MP phone cam to be equivalent to an 8.0MP point and shoot. (After all, "they're both 8MP")