They do get sued pretty often, but the law is on their side. The more absurd the circuimstances the easier it is to get away with as Parody, the only time you’re in hot water is if the viewers in any sizable metric would be fooled into thinking the things portrayed in the show were true when in reality they weren’t. In fact, when South Park revealed everything about Scientology’s internal beliefs they were almost sued but Scientology backed down because what South Park said in the episode was actually verifiable truths that could be backed up with evidence in court.