This is a very common question and the answer gives an insight into how mind works.

The following explanation applies only for the unconscious-mind (aka the fast-thinking mind from the seminal book by Daniel Kahneman):

People are used to searching on the Google-Homepage so much that they forget that the search is now integrated into Browsers and Computers.

So what happens is that when someone wants to search for information, the first thing they look for is the interface that they have used a lot (which is Google-Homepage). Now fast-thinking-mind tries to go to Google-Homepage.

So they type "google" into the browser search bar (which is what the unconscious-mind does when it is looking for some webpage) and Browser-search-bar-Google opens Google-Homepage.

The end result is you see a lot of people typing Google into Google search. However what is actually happening is that people are looking for Google-Homepage into Browser-Search-Bar.