That's like saying laptops have barely evolved since they weighed 15 pounds. iOS has evolved a hell of a lot since 2007, and it continues to do so. Just because Apple hasn't slapped new paint on it every other year doesn't mean that it doesn't continue to have new features and improvements every year.
Just about everything you see in apps today is impossible on iOS3 (or absurdly impractical). Even something as simple as the ubiquitous slide-aside menu would have been insanely hard to implement in the first iOS SDK release.
Since the SDK first landed we've seen Grand Central Dispatch which has helped apps become way more responsive than before. We've seen large improvements to Core Data that helps apps cache more intelligently and easily. MapKit didn't even exist in the original SDK. The list goes on, and on.
This is a classic "frog boiling in water" phenomenon. You don't think there have been any real improvements to iOS because you've grown along with these evolutionary changes. If I gave you a iPhone 3G with iOS3 and a iPhone 5 with iOS6 - along with common, well-developed apps of their time - you will see and feel the difference immediately.
Meanwhile, the Android frameworks seem to be crawling and it seems like no one can be bothered to create a decent IDE for making Android apps.
iOS 6 on the other hand offered practically nothing in terms of lower-level apis, or even bugfixes for bugs that have existed for a few years.