Because the richest company in the world can't afford to have more than 5 different laptops in the showroom or what?
If they would have updated the Air with a better screen, I wouldn't have suggested dropping the Air. It just seems that they have no interest in putting a decent screen on the Air (I'm not talking Retina, even an IPS panel would be an improvement), or making any real improvements to it.
Not only that, but the MacBook really seems to be out of place price-wise compared to the MBP. They really need to move it down the price scale and it wouldn't make sense having the Air and the MB at the same price. In 2018, the rMB should be Apple's entry level laptop. It really is the new Air in my mind...