That's because its not. I can't even tell if I'm running 4.1/2/3/4. I have a Nexus 5.
Contrast this to iOS - whenever there is a new iOS version, developers have to scramble to update their apps because they very often break or the UI is broken by changes, etc.
Android is much much less hassle when OS updates roll around, and I develop for both.
[Rant]: I find that it gets a bit silly in iOS land, in the sense that local (South African) devs who have abandoned/flopped apps had to go and update their apps after forever to match the 'look and feel' of the new iOS, whereas with Android one can go without noticing that.