Cookie consent banners and such come from the ePrivacy Directive, not the GDPR. The banners themselves were never mandated, but lacking any other standardized opt-in signal, that's what everyone converged on anyway.
A lot of sites put up the banner even when they're not serving anything but "essential cookies", just as a CYA mechanism mandated by legal. And to some degree, I can see legal's point: the site might be just fine now, but you just know somewhere in the sausage-making process, someone's eventually going to toss in a dependency that brings in a tracker without clearing it first, and boom, exposure.
Having a clear non-interactive signal that's legally recognized should go a long way toward clearing out those annoying banners.