There is a natural assumption of truth, or a truth bias when humans communicate with one another. In other words, when we're listening to others or reading their words, our automatic assumption is that the other person is telling the truth. This usually works out fine. If you ask someone where the restroom is located or if it's raining outside, you can safely assume that most people will not lie in their responses. Imagine how difficult it would be to converse with someone if you assumed that everything they were telling you was false! Indeed, questioning the truth of a statement and then choosing not to believe it requires additional mental steps. For the most part, humans are "cognitive misers," which means we typically don't expend more mental effort than seems necessary in a given situation. It makes sense then, that when we see something online, even if it is fake, our default is to believe it, at least at first.