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.

You don't need cookie banners if you don't use invasive telemetry.

A website that sticks to being a website does not need cookie banners.