Let me explain why with an example: say you're the type of people who doesn't care about "privacy" online ("I've got nothing to hide"), or you do; and you want to "support " certain ad-supported websites you're a fan of; but not that new clickbait toilet paper your aunt sends you.
I can't think of any way to have a good UX to opt in or out of "tracking" cookies which people would actually use (few will bother changing the defaults, and most mindlessly click ok).