Because they are fighting the people that absolutely do not want to see advertisements whatsoever (most normal people I know do not have ad-blockers), which already makes them the least likely to be attracted by advertisements. If anything, those brands are souring their reputation anytime they are seen by those kind of people in a medium that those consumers find obnoxious.
You can make a similar argument that the RIAA/MPAA going after piracy is a waste of time. Again, focus on delivering value to actual customers.
If Facebook spent more time on making a friendly ecosystem / community I would be more open to signing back up again. Instead, it seems they are hyper-focused on advertisements at the expense of everything else.