Because they collide. GDPR regulates how to work with private information and makes it absolutely clear that cookie popup informing the user that a cookie was already set is not sufficient. But if the cookie is not used for privacy relevant user tracking, then a popup is not necessary in neither regulation.

Those cookie popups have been a misinterpretation in the first place, the general recommendation for them already redacted by the one data protection agency that first formulated that consequence as requirement based on the regulation from way back then. They are completely out of date now. Note also how the site you link does not require them - it instead completely blocks the site when cookies are not allowed by the user, which is a different beast.