As others have pointed out, US first amendment laws generally protect shows like South Park because it's generally understood that the characters in the show that resemble real people are parodies, and the show runners aren't stating a fact that the real person said or did a thing in reality.
Funnily enough, the UK has much stricter laws about defaming people - the country has a strict class system, and it wouldn't do if poor people could embarrass rich people - there is a significant carve out for "vulgar abuse". If I was to go on TV and (for sake of example) called Boris Johnson three shit-stained jugs of fetted piss wearing a trench coat, that would be ok, because people understand that to be a euphemistic insult, not a literal statement of fact. If I went on TV and said that he was a drunk, that wouldn't be - unless I can prove that he is an alcoholic, he could sue me for libel. The outcome of this is that an equivalent show to South Park could be made in the UK, it would just have to be utterly filthy
In Penn & Teller: Bullshit, they did something similar. They pointed out very early they would be more vulgar than most people expect. This is because words like fraud, quack, scammer, etc were specific allegations that could land them in court. But words like asshole were not, and were much safer.
Nobody said the US doesn't have a class system? Also the person you're responding to appears to be a kiwi, not an American.
Non-Americans making fools out of themselves while trying to casually denigrate the US is one of my favorite internet traditions. It's especially entertaining because there are plenty of valid criticisms, but people often seem to go for the most lazy, inaccurate generalizations and reveal their ignorance.
Our class system is based entirely on how much money you have. The UK still has a legal aristocracy based on how much land your direct ancestor owned 800 years ago.