When I've seen discussion of these in the past, belief is often connected to fear or guilt stemming from how bodies react differently to exposure and/or infection. There's an element of randomness whether you breathe in enough of a pathogen, or run into a super-spreader. It can be a relief to think that people are predisposed to do better or worse, and either you were already immune or you can control it.
At the risk of playing devil’s advocate, I think it is very weird how differently different people responded to Covid. Some people barely got a cold. Others died. I really don’t think you can write it off to “well ${dead_person} just inhaled more of it”.
But that's the case for literally every infectious disease. We all have slightly (to dramatically) different immune systems and other systems. Even without being immunocompromised, some people die from seemingly innocuous diseases while others survive the most deadly diseases there are.
Even Black Death killed "only" an estimated 50% of the population. That means that there were necessarily a lot of people that got infected and survived it, probably some of them asymptomatically. Bodies are complex and varied, and there's no 100% way to predict how they'll respond to any given situation.
The book I was just reading says that the genes for MHC (Major Histocompatibility Complex) molecules are among the most diverse genes in the human population, with even the suggestion that people find others with different MHC genes than themselves more attractive.
That's no different that with most other diseases, though. Everyone's body is different and has different levels of resistance to different things. The same disease often affects different people to different degrees.
The most dramatic examples of this are when children start going to school and bring home various colds and such. Some of them barely affect the children at all, but will leave the parents bedridden. And some barely affect the parents at all and leave the children bedridden.
