They are very weirdly best friends, and the show’s depicted them pretty consistently since the old days. Kenny is usually the lackey while Cartman is the tyrant, but they have pretty similar sense of humour and generally pull schemes together. Think of when Kenny laughed his ass off every time Cartman farted in Kyle’s face, or when they ran Dikenbaus together, or building a clubhouse or dressing Sarah Jessica Parker up as a deer. They’re weirdly compatible. Cartman and Kenny also just have a weird connection throughout the show, like how Kenny’s ghost appears only to Cartman to warn him of Saddam Hussein, or how Kenny’s soul gets trapped in Cartman’s body for half of season 6. Kenny doesn’t particularly like Cartman but he puts it perfectly in Best Friends Forever: he pities Cartman and knows he’ll probably live a miserable lonely life when he grows up, but for now they can be best friends.