Because the incentives are not aligned. The banners are there not because they are required, but because websites want to badger you into agreeing to tracking.

Because the incentives are not aligned. The banners are there not because they are required, but because websites want to badger you into agreeing to tracking. Websites don't need to show a banner if tracking is opt-in or if first-party cookies are only used for functionality not tracking! This is why even though DNT exists, nobody respects it. The point is to make it annoying so people cave in to agreeing to allow tracking. Any standard that is not "by default allow tracking" will not get adoption from the site owners because it reduces the tracking they can do. Obviously, the whole point of the EU cookie regulation / GDPR is to not have tracking by default (which is unfair to the user), but at the same time, being a regulation, it also doesn't want to default to "no tracking until opt in" as that would then be unfair to the sites. If you can't allow by default and can't deny by default, the the only remaining option is to ask. This is really a no-win situation.