for Gmail, you have to log in. It has to know it is you. It has to know that everything it is serving you is only for you. It has to retain copies of documents that -- as far as it knows -- are unique to you or whomever you share with. It has to do all this while keeping that information safe from other people who might want to take a look at it.

And that's just the items I can think of in real time while typing.