Antifa operates in a grey area, where they go up to the borderline of what is legal. That makes it hard to take action.

The people funding Antifa do it through multiple layers of shell corporations/charities. It's very hard to make a connection to the true backers. Even if you could, they will just say "I didn't know my donation would be used for that."

The moment you do anything, they will lawyer up and fight it every step of their way. They own a substantial number of judges. They will drag any action out for years. Eventually, there will be another Democrat President who quietly drops the charges. If you get a conviction, they will still tie it up in appeals for years, and the next Democrat President pardons them on day one.

There are rumors of people who knew Charlie Kirk was about to get assassinated, and were bragging about it before it happened. That still makes it hard to prove anything, because they could just say "I was speculating/hoping and got lucky."