The refrain that Omicron is "mild" is being parroted by people who don't understand what they are talking about. In fact, the Omicron variant (like other COVID variants) is relatively mild among individuals who have been vaccinated.
That hasn't stopped the vast majority of folks yeeting themselves off a dunning-kruger cliff and assuming that they are competent to discuss the topic because they read a medical journal or an opinion piece that aligns well with their own perceptions of COVID and the broad spectrum of economic, political, and medical topics (myself included sometimes).
That's not the only variable. IIRC, South Africa's population is on-average much younger, less obese, and much more likely to have had a previous infection compared to America's.