I would guess because there are also 100 different android GUI's that manufacturers put on top of android and every device acts differently so they have to make sure it works on them. I remember my biggest pet peeve with android was that some android apps didn't work as well on my incredible as they did on other android devices.
This, my friend develops apps and this is his biggest complaint about Android. Its just much easier for ios devices. Sucks for us Android users though.
Interesting. I had read that these days developers create an Android app first then switch it to iOS instead of the other way around which is how it used to be.
Also, my wife and I have two completely different Android phones and all the apps work the same.