Oh, it's because they're a monopoly. Google has a lot of products that people have to use because of the dominance of other products, platforms, or standards that Google controls. So as long as global growth is strong and more people are using the internet, Google will grow amazingly.
If, on the other hand, Google were in a competitive environment, you'd see it growing depending on whether its products were getting better and better.
But as observed in this thread, Google search is entirely ads above the fold. Gmail is becoming a Google controlled proprietary protocol. Etc. etc.