Because the trend of buying desktops is long gone, but the need for SOME people (not you) owning two distinct machines is not.
As capable as the air may be as a solo system, it does have some severe limitations in terms of storage and upgradability. So it's perfectly reasonable for a person to buy a beefier notebook workstation and a slimmer ultralight for travel.
I never said me. I have two machines, a MBP and a Air for travel. Because I bought a Air last year and the 4GB of ram was limiting. So I gave it to my wife.
Bought another maxed out Air this year, well almost. i7/8/256, and use it for travel. Also bought a Retina but returned it for the Air because of the third party support for the screen. The Air limitations are gone as far as I am concerned. 8GB of ram is good for most usages. Processor is fast, and the SSD speaks for itself. Out of the box a i7 Air will outperform because of the SSD a MBP is most tasks that are not only CPU bound. Now why would you have both? A Air can do prety much what ever you throw at it.
The same limitations you speak of are the same for the Retina. And I did say 'most' not all people. The majority of people using a Air will not need another machine.
Storage, 512 GB, i7, 8GB of ram is good enough for most people. Upgrades, well that is the price you pay of its thiness. Macbooks going foward will have the same limitations.
The thing is that even though I have my pick of what ever I want to use at the office, Mac Pro, Imac, MBP, I usually use my Air. Just because it is that good. It is a powerfull machine. If you need more space buy a external, more GPU and CPU power buy a MBP or Mac Pro or Imac. No need for both.
Most people are not business owners, and do not have my usage needs. There really isn't a need for both. Redundant. Buy what you need. Why waste money?