People's immune systems vary greatly. A perfectly healthy person who never gets sick may be particularly susceptible to a disease like COVID-19. It's just a dice roll. You never know. Sure, probabilistically, an athlete who eats well and trains well is an extremely rare case for COVID-19 deaths but they likely just had a rare combination of immunological factors that resulted in them being more susceptible to SARS-CoV-2. Also, even with a perfect immune system with an ultra high titer count, ready to fight SARS-CoV-2, if you get an immense initial load of the virus, your body will not be able to fight off the virus fast enough and you will die. The viral load you experience in the first 4 hours of exposure is the key to living or dying. Same with any disease.