Because it is a convenient, objective property that is easy for non-knowledgable consumers to understand.
They are right that megapixel quantity doesn't automatically make a good camera. But that doesn't mean more megapixels are completely useless. It's one factor of many. How you weight that factor against the others depends on your photography goals.
Just as an example... I often use my phone to take shots on construction sites, for reference back in the office. I might take a picture of an equipment data plate or an electrric panel's breakers. In this case, megapixels rule the day, because my usage is to read the fine print once I'm back in the office. I don't care about acurate color, low light ability, a fast shutter, or any other criteria in this case; just fine detail. Now I do realize this isn't a common usage for cell phone cameras among the mass of consumers, and neither is it my only usage, so I hope you understand I'm not dissing the One's camera. Just illustrating my point.
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