These projects continue to exist because no company has felt it's in their interest to bring suit, I guess. This is the type of thing that companies like to keep quiet, and if something like uBlock Origin isn't a pervasive threat, they won't risk publicizing and potentially losing the loophole. In particular it would be dumb to sue the EFF for Privacy Badger, since part of the reason the EFF exists is to fight such things in court.

Look up the "RAM Copy Doctrine". Basically, it means that every time data is copied within the computer's memory, it's a potential infringement. The HTML source of the page you've downloaded undoubtedly qualifies for copyright protection. Your argument would be an implied license to modify the page to make it suitable for display, but standard ToS will typically forbid any type of modification, removing any ambiguity into whether third-party extensions that modify the page may have an implied license. Your license would typically be limited to displaying the page as transmitted, and allowing an extension to read and modify the page would be an infringement.

I'm not a lawyer but I had a neat little SaaS business that was killed by a C&D along these same lines. Multiple attorneys reviewed and this is basically the summary. When I suggested that we do a peer-to-peer data transmission thing to avoid handling or transmitting any copyrighted content, I was warned that doing so could easily be interpreted as conspiracy and that it'd be best not to go down that route. Maybe someone else's attorney would say something different, but that's what mine told me.