You are asking the question from the wrong side.
The right side is - why those bureaucrats are so stupid, short sighted and people hating that we have the frustrating banners even on harmless websites.
I don't use a single cookie in my landing pages or product, but I still have the cookie banner because so many of the marketing tools track through cookies.
Removing cookies would reset consent and preferences, violating regulations and frustrating users with repeated banners
Analytics, marketing, etc.
User-preference cookies (e.g. light mode vs dark mode) are not "strictly necessary", and therefore probably still require consent under GDPR
Likely metrics and tracking of traffic flow (origin, dest, etc.)