They aren't changing minds. They're reinforcing and radicalizing the beliefs of people who are already in the bubble they're in. And it's far from useless!
Try to think back to the last time you saw an ad that was (1) actually directed at you, and (2) trying to change your mind. I can't tell you the answer for me, because I never see ads like that. Harris ran her campaign on that last fall, and it failed; partially because people have been lied to through screens often enough that they tend to only believe things that screens say when they're saying stuff they already believe.
As you already know, we'll do all sorts of mental gymnastics to make new facts fit with old opinions, probably because there was some kind of evolutionary advantage. The fact is, most people have already made up their minds. Changing those minds is hard, because people have a visceral reaction to facts, ideas, and opinions that go against their pre-existing beliefs—just like what's happening to you right now. It can be stronger or weaker depending on how important the belief is, but in either case, the only time we're likely to change our minds is in person; again, probably due to some evolutionary advantage (pack bonding, maybe?). Not online, not via broadcast, but in person; usually in small groups or even one-on-one (I've seen some evidence, though I can't find a link, that voice and video calls—but not text messages—are almost as effective as in-person discussion).
Protests gain momentum when more people change their minds, and people change their minds when the people they know and trust and respect go to protests. It's not the only way that fascists are overthrown, but it is definitely the least-bloody way.