It's by design because the vast majority of people don't care about that information and it makes the website worse for them to have a big ToS banner at the top of your page.
I don't think many websites have a secret ToS that they hope you won't read, I think most of them don't even know what their own ToS say. I signed my lease on a site with an explicit checkmark for ToS that said I agreed I would only use exactly IE7 to use their site.
Even for sighted people, the notice is often easy to miss - and this is by design.