This might help to explain why they have difficulty accepting any evidence which is based on complex modelling and more abstract methods.
In a similar way to an insistence by flat-earthers that the earth is flat because it LOOKS flat from the visual perception of the human eye, the no-virus folks deny the existence of viruses because they cannot directly purify, isolate, and visualize them. They are absolutely bound to materialist methods and interpretations of the data. If we consider that some of them might be OPs, then it would make sense that they may not actually have the capacity to think of it in any other way.
Seems to me that a good ability to grasp abstract concepts requires a good ability to think outside of our own 'box', i.e. less subjective and more objective. In this respect, animals are rather 'self-centered' compared to humans as a function of their perceptive abilities. The less you are able to perceive, the more you are required to rely on what something seems like/means/feels to you rather than what it actually IS. So yeah, maybe we're talking about a fundamental inability here.