About "natural" immunity.
You might find this surprising but the immunity of those who have had active infections is relatively temporary.
Why? No one really knows for sure, but this virus is in the same family as cold viruses. And for some reason, your immune system either doesn't take them very seriously, tends to forget them quickly and/or has some means of evading prolonged immunity.
The following is a bit speculative but it appears that in populations such as some east asians where similar viruses are known to be endemic that there have been genetic changes in the population that appear to give some modicum of immunity.
Whatever the case though, even with the rush of antibodies caused by the Pfizer and Moderna vaccines, your immune system STILL tends to forget the virus very quickly to say, something like polio that your immune system remembers forever.
That's why you keep hearing about boosters and annual flu-like vaccines.