Because you think 20MP quality in 7mm thick module is possible... Go read your physics book and come back to me.
There are already 16MP and 20MP sensors in cell phones and the 6+ already beats them. Because 8MP is adapted to the size of the sensor you can put in there with a thickness X.
Apple could go slightly bigger, but that would mean adding 1-2mm to the thickness of the module. They'd have to make the phone thicker at the top than bottom for that.
----------
Oh, really, they can bypass physics too? They can invent light from thin air?
----------
All reviews say different. You better google more and better.
I have not seen any reviewer taking a picture like what I said. You can try it by yourself. Try take a picture of someone with bright background light.