Because the US government wants to maintain access to FBs data. Shutting down FB would be a big blow to surveillance, hence they will never do anything serious against it. They’ll just put on a show, scream publicly in outrage about what FB does or whatever, and then nothing substantial will come out of it, because they never intended to do anything in the first place.
The regulators who are supposed to enforce the GDPR are either incompetent or unwilling to do so. I suspect there might be political problems with stepping up enforcement considering a lot of politicians rely on social media & ads (which is powered by non-consensual data processing) to help with their (re)-election.