You do remember Windows was born as nothing more than an 'addon' to DOS right?
That may well be, but it was all project managed and delivered as one 'windows' project.
It really is not the case with Linux, every single bit of a Linux distro ranging from the Kernel upwards is a series of independently developed and maintained packages coded with different people with different coding styles, ideologies etc.
All the Linux distro disk does is say ...
'here you go.... one disk that will install Linux kernel and a bunch of CLI/GUI tools for you without you having to spend your time going out collecting them all and compiling them yourself'.... kde/gnome or whatever desktop they choose to bundle is just a.n. other package sourced from elsewhere that they've just added some fancy graphics to in order to make it look pretty.
Also, have you heard of Android? - That seems to work alright for just a few non-technical users.
Android is actually a purpose built integrated system with a specific embedded purpose in mind.
You can't compare it to a Linux Distro' which is basically a bundle of Linux software readily available elsewhere and the only real difference between "desktop" Linux distros being the degree to which they try to make the GUI look all fancy.
The GUI in a linux distro still remains independently developed and that is why the degree to which it does/does not tie in to the backend remains patchy and at times painful and scares the average user off because they need to resort to CLI for this or that.
I've used Ubuntu 14 Desktop, its nowhere near where OS X or Windows is today, it lags behind.