I really don’t understand why the desired cookie behavior could not be set globally for all websites in the browser. Why do I have the accept / reject them on every website and trust the website will actually do what I ask.
It can be, see Global Privacy Control [1]. As an example, the Complianz consent plugin for WordPress can detect and respect the user's GPC setting, but that feature can be toggled. So even though this exists, many sites will still ignore it.
Because society is full of dickheads? In a just world, it would be legal for a policeman to bludgeon anyone who does not respect a user agent's <https://en.wikipedia.org/wiki/Do_Not_Track> setting. Second offence, hang them from the flagpole in front of the parliament building. The problem would disappear over night.