You’re surprised that following the scientific guidelines to minimize the chance of catching the virus has worked?
In any event, it’s well established that the great majority of people who catch the virus don’t get very sick, or are completely asymptomatic. So you don’t know that you have been virus-free.
Why is there so much variance in response to the virus, even after taking into account risk factors like age and other preexisting medical conditions?
(1) Genetics. This may include such things as variants in the cell surface proteins that the virus interacts with; blood type has been implicated; and almost certainly HLA haplotype will be a factor, an aspect of the immune system that has evolved to be highly variable in the popluation, for the express purpose of ensuring that a pathogen that can evade one person’s immune system cannot necessarily evade everyone’s immune systems.
(2) A significant proportion of the population may have acquired some degree of cell-mediated immunity due to prior exposure to other coronaviruses.