I don't know if I'd characterize it as an inability to understand. This is sometimes the case - particularly where people are unaware of their foundational assumptions and how that shapes their worldviews - but more often than not it's mostly a product of worldviews being different from one another. If you're using map A to interpret the territory, you'll navigate it differently than if you're using map B to interpret the territory. That sort of thing. Some folks are good at using multiple maps or paradigm shifting, others are not.
Speaking as one of the non-theist types, I think you make an important point here.
However (unfortunately) there are plenty of people unable to make room for other belief structures.
Somewhere along the way, I feel like there is an ever-increasing level of literalism in the way we communicate which seems to me to be quite limiting in allowing complex conceptual discussions around topics where there is more to be considered than just a binary position.