You are confusing URI with URL. A URI is just an identifier. Just because it looks like a URL doesn't mean it is a URL that locates a resource. It's just a name.
URI names could just as well have been something like "asdkjfadsjfgjasdhgfkjhdsagfjkhgdsajfha" or "Fred", but the format of a URL was chosen because by including a domain name in the URI, it's easy to avoid collisions. For example, you should never create a URI with google.com in it becaue you don't own that domain name. So therefore, your URIs and Google's URIs can never collide.