Why does covid19 very often cause no symptoms at all but can also kill a very healthy person?

I don't think we have the exact answer to that question yet. It very likely has to do with any one person's immune system. It could be that someone who appears physically healthy has some underlying condition previously unknown, or that the person was psychologically stressed which we know can affect the immune system. There is a lot we are still learning about this virus. Keep yourself healthy!

Excellent question which puzzles superb doctors. Biological variations in immune responses can cause star athletes in terrific condition to become disabled, and seemingly healthy children to have critical response. But we cannot predict vulnerability in a large population. COVID-19 may well be a disorder of the vascular system in some, and respiratory system in others.

I agree with Dr. Maes that there may be underlying medical conditions that one wasn't fully aware of. I would normally not expect someone who is completely asymptomatic to die from COVID 19.