And practically any company can build as many billboards as they want. Online ads aren't restricted by physical space. Google just happens to own most web traffic because they're really good at search.
Maybe a better analogy is Google owns the biggest road system (search) and all land beside it for billboards (ads). Very few people use other road systems.
Now, one might say "But users can choose other search engines." But users are the resource, not the customers. Right now, Google controls most of these resources. And the very nature of search means they're not sharing access with competitors.
Now, one might say "But users can choose other search engines." But users are the resource, not the customers. Right now, Google controls most of these resources. And the very nature of search means they're not sharing access with competitors.