Because resolution is probably the least important from all the characteristics of a camera (and it can even go against quality, a larger resolution without an accordingly better sensor might just as well give a worse overall quality).
In the usual circumstances of phones, probably the most important factor is the sensor size and quality, and neither will be accurately described in sales literature. The physical size of the sensor has obvious influence on image quality. A smaller sensor will see less light, and while the attached hardware and software will try to compensate, this will increase noise.
Then come the actual lens and optics. This is again something you can't really describe and compare in specifications but it matters a lot. Phones that excel in the camera department often have more complicated and more expensive lens systems coming from reputable old names of the photographic era.
And finally, the processing of the image. I don't think you can find any phone camera these days that doesn't do an awful lot of processing under the hood because that's what people want to have. Most people are very bad photographers so it would really upset them to see what they actually shot. :-) Phones compensate for almost anything these days, lighting conditions, movement, they enhance contrast, sharpness, faces, whatever.
So, what it boils down to: there is really no other way to compare two phone cameras but to shoot and compare for yourself. If you're lucky, somebody else already made the comparison and you can find the pictures on the net. But always judge by the pictures, not by specifications.
