Because this is there 1 millionth cookie banner, because every site and their momma has one.
Also, 90% of cookie banners are not this good. They tell you nothing, hide the "reject" button behind multiple screens, etc. At that point the consumer is trained to click accept.
This is the definition of informed consent
However, since we are discussing the banner that The Guardian website shows to US viewers, I assume they’re trying to comply with California privacy law, which does allow opt-out regarding the sale of personal information.