Because it makes no difference to my assurance-level which button I click. There's no way of knowing what they do serverside with your form submission (and it nearly always is a form submission).

Cookie approval has to be under the control of the user, not the website. So it has to be done by the browser or an extension. So if I have user-controlled cookie-approval, I might as well click "OK" on the form - the site might treat me better if I do.