> I don't understand why we won't stop all this nonsense and build this stuff into web browsers as opt-in or opt-out (let the user decide) and therefore ensure that the UI is always the same without any dark patterns.

Because GDPR covers the everything, not just the web. If we had protections in the browsers, smartphone apps would still be affected. Instead, we fix it on a regulation level and no matter if it's web, apps or quantum-apps, we'll be covered.