Because AFAIK there's no standard way to identify them because each website comes up with its own design...
This is all rather silly, given that the choice to allow/refuse cookies, and the prompt, could have been better implemented at the level of the Web browser...
The cookie warnings are about refusing cookies, which is something that is completely up to the Web browser.
Specifically, Web browsers could warn when a website sets a cookie and ask for user consent before storing it (and if the user does not consent then the website becomes unavailable).
