Do you know that the French Academy declared sometime in the 17th Century that nothing new is left to be invented?
That's the problem with ordinary thinking people. They cant see beyond the existing products/services. You have to think radically to invent products or services or ways to do things. The iPad was not a true invention (tablets were available before) but it allows people to do things in a more convenient way through an new intuitive interface and allowing people to add functionality through Apps.
And it is not true that we do not have inventions in those categories you mentioned -
OSs: OSX Lion is radical, Win7 may not be radical but have many good under the hood innovations,
Cars: Do you realize that Hybrid cars (runs on Gas & Electricity) are an invention? Have you heard about Google unmanned self driven cars? Isn't that innovation?
Off course products can look similar but when you start with a picture of a competing product and ends with the same product from the assembly line, you have some serious problem.