The actual "easy" part is that you can sue anyone for pretty much anything. Suing is entirely different from winning the case.
Why they think they have a chance of winning is the weirder question, especially when Musk publically told the advertisers to go fuck themselves.
Well first of all, xkcd.com/386. And also: because I don't like fascism or promoting the spread of fascism.
There's a saying that reading history may help prevent repeating the mistakes of history. You should try some of that.
To reiterate: the reason to be anti-fascist has absolutely nothing to do with how good they are at doing things and I don't see any good reasons to hide this fact.