For one thing, they have a policy of only showing advertising that is “relevant,” and you can’t think about relevancy until someone types in a search query. For example, if I type in the search, “Dogwalker rates in Washington DC,” Google may decide to show me an ad that links to a local dogwalking company. But until I type something into the Google search bar, Google doesn’t know enough about me.
But another reason is that Google’s brand is heavily wrapped up in its clean, ad-free homepage, and it probably doesn’t want its homepage to turn into something like the image above (which would make Google feel like all the other search engines). Did you like reading this blog post? Sign up to get my new blog posts delivered by email by clicking here.