That's why such agreement wouldn't be legally binding in a big part of the world. The users in turn know it's more or less safe to click "accept all" on a random website.
I think most click "accept all" because they simply don't give a fuck, not because they understand some judicial situation. Most (common) people don't understand most things on the internet, even less if it touches law-related stuff.
They don't give a fuck because they know no case when clicking "accept all" on a random website got someone in trouble.