One difference between third party cookies and FLoC is that with FLoC it is being explained to the public how browsing history is being used. The third parties who send Set-Cookie headers do not write up documentation attempting to explain to the public what they are doing and why it is not a threat to privacy.

As other comments point out, it would be more difficult for people to "be all up in arms" about third party cookies because third party cookies is not an issue that is easily attributable to one entity nor one set of documentation.