> many browsers already forbidding third-party cookies
1) this doesn't prevent marketing parasites from hoping that some people don't block them and attempt to set them anyway
2) data processing consent covers more than just cookies
> many people using only essential cookies
See 1) above - until marketing people accept their defeat and adapt, they will push for implementing a consent management solution to allow people to opt into more than just essential cookies even though nobody in their right mind would do so unless forced
> many others removing cookies with extensions such as Cookie Autodelete
That wouldn't be valid as per the GDPR. You can't rely on an optional browser extension to make your site GDPR compliant.
> where do these companies get profit from?
Considering implementing consent-management is trivial for any Javscript developer (and I'm sure open-source solutions exist), the only remaining option would be of a legal "shield". The vast majority of these solutions aren't actually GDPR-compliant (the GDPR explicitly forbids dark patterns, pre-ticked checkboxes, etc) but actually complying with the GDPR would either destroy the client's (unethical) business model (if they are ad supported) or demonstrate the uselessness of their marketing department (as sales keep being made despite none of the tracking working), so shielding themselves behind such a company allows them to break the law while being able to blame it on the CMP should things go south.