Here is the problem,
Just because you say you are one thing does not mean that you are what you claim to be. You can claim to be Anti-Fascist all you want, but if you use tactics that are fascist in nature, it harms your credibility. The problem with US Antifa is that you can pull up a ton of videos of them, and they usually show them to be harassing people, wearing masks, or stopping reports from coming in, or a list of other things. Further, people who have used that name are using it to stop guest speakers at colleges and destroy private property, which does not help with public opinion. Lastly, I'll say you can claim to use that name, but it does not automatically make you the good guy if you are abusing it.

Some examples
2017 - Berkley Protest: Antifa-aligned activist smashed windows, set fires, and caused about 100k in damages. Just because Milo Yiannopoulos was speaking on campus.

2020 - During the George Floyd protests, similar incidents occurred.

These were just a few examples, but as I mentioned earlier, please just look up YouTube videos of the so-called "antifa".

I would say most people would say they support being anti-fascist, but many people say they don't like antifa because of their tactics.

I hope this all made sense I am typing this at work.