The biggest problem, and I do think noise747 has a point here (but it gets lost by going so OTT on other things) is that iOS and Android - and other OSes and platforms - are now so competitive that it's considered acceptable and beneficial to keep changing things, not just adding new features. A new look gets people talking and taking an interest again, even if the changes are visual changes for change's sake.
So iOS 7 became a totally radical overhaul, that looked a bit Mickey Mouse to me (naturally, many love it), and now Android has once again changed its look massively - after various changes over the years (Android 3 for tablets being one particular big jump that some people loved and others hated).
The problem is that if any of these OSes or apps ever got to what most people considered to be perfection, it wouldn't just end. It would still have to change, and new thinking at the firms would no doubt see someone decide that everything we loved was 'wrong' and now it's time for something new. Much of the time it seems to be down to copying what a rival has done.
Some people love that feeling of something being all new and get excited by it. Working in technology, I certainly don't have any problems with it. But I know my mum gets upset if I move around some icons on her Windows desktop, and expect there are many other people who will do the Android 5 update and be very confused.
Perhaps the Nexus 5 didn't sell massively to 'the general public', or indeed the N4 or N10, but both the Nexus 7s did.
I updated my N5 and N7 via my desktop so don't know if the OTA update results in a 'what's new' tutorial?