Because the megapixel value is only one small factor in the process. The sensor size, sensor type, lens, exposure control, processor and software altogether set the quality of the image, not the number of dots in it.
However, you don't see "Digic V Processor" advertised too much as the buying public doesn't relate to it. They see "8" on this box and "12" on that one. 12 is better than 8.
People who do know the difference aren't influenced by big-letter marketing techniques. They also don't care too much about a phone's pixel count. Nothing with a lens/sensor combination that small is going to produce anything decent anyway.