Because this is malicious compliance by websites. They are attempting to annoy people into clicking the easiest button, which is always the "store all my data, spam me at will" button.
And then the second is: browsers don't really know the purpose of any particular cookie, or how much the user actually wants it or not. Due to how cookies work, they really have no way to know. Cookies are not standardized enough.
For more than ten years I've had all cookies turned off by default in Chrome's site settings, and I click two buttons when visiting a site if I want to allow it to store data on my machine. That allowlist is persistent so I don't have to think about it again.
Cookies for managing login's are explicitly allowed by the GDPR. If you get a cookie choice prompt when visiting a website, it is an indication that the site is placing "advertising/tracking" cookies, for which the GDPR does require consent.
Without obtaining consent or having a legitimate (i.e. functional, not economic) reason, the website operator cannot collect server-side logs or fingerprints either. Or they might not need consent to collect data (e.g. remember your purchases for refunds) but do need it to disclose that data to third parties (e.g. to feed the purchases into a recommendation engine... or, let’s be real, an advertisement profile).
None of this nuance is enforceable browser-side. It could in theory be communicated in machine-readable form by the browser, such as with a DNT header, but before somebody sues over that I doubt it’s going to be honoured.
(I remember that the SameSite cookie proposal had a follow-up, even more web-breaking same-origin-policy cookie proposal. That’s probably the most meaningful thing you can do client-side. But it had seemingly died when FLoC did, and I can’t find it now.)
For example, load StackOverflow in Chrome incognito. It has buttons for "Accept all cookies", "Necessary cookies only" and a "Customize" menu that gives you checkboxes for "strictly necessary", "performance", "functionality", and "targeting cookies" all with a lot of links and explanations.
It isn't just a matter of turning off third party cookies.