Uncertainty terrifies them. Radical skepticism is more palatable when couched in euphemisms such as 'provisional truth within my model'.
All good arguments are broadly circular.
The more of reality encompassed by the circle, the better the argument. In presuppositional apologetics, we show that our circle turns on God as its center and it encompasses all of experience. We then show that the unbeliever's circle, which turns on human reason as its center, explains nothing at all. In fact, as the unbeliever's argument progresses, it leads to conclusions which undermine its starting point. The beginning and endpoints cannot be reconciled, i.e. the circle can never be completed.