Because they track usage stats for site development purposes, and there was no convergence on an agreed upon standard interface for browsers since nobody would respect it. Their banners are at least simple yes/no ones without dark patterns.
But yes, perhaps they should have worked with e.g. Mozilla to develop some kind of standard browser interface for this.
This is actually not true. I just read the European commission's cookie policy.
The main reason they need the banner is because they show you full page popups to ask you to take surveys about unrelated topics like climate action. They need consent to track whether or not you've taken these surveys
Their banner is just as bad as any other I have seen, it covers most of the page and doesn't go away until I click yes. If you're trying to opt out of cookies on other sites, that's probably why it takes you longer (just don't do that).
They create profiles of visitors that allow them to, e.g. through polls.
It's usually a click or two to "reject all" or similar with serious organisations. Some german corporations are nasty and conflate paywall and data collection and processing consent.
But yes, perhaps they should have worked with e.g. Mozilla to develop some kind of standard browser interface for this.