It's so simple: the browser should expose a user configuration setting, which can be used by any website to automatically answer whatever question the consent banner thinks it needs to ask. At the highest level, three settings: Ignore, Reject all, Accept all. More fine-grained settings could be standardized (conceivably), although I would be surprised if many people cared to use them.
This is one of the first ideas people tried, more than a decade ago before GDPR was even a thing. The Do Not Track header was proposed in 2009 and implemented only in Firefox. The problem is that advertisers don't want this solution, because they want you to accept all cookies and a single browser-wide config setting makes denying way too easy. so they just ignored the setting, and nothing ever came of it.
