The simplest explanation is the best explanation, as adding factors that may or may not exist “reduces the actual possibility”. Applies to a lot of theories, for instance, those about science.
Mathematics and logic support it; each time you add a probability that is not 1 (guaranteed) to an “AND” condition, the probability of all elements being true will decrease. Most things being doubted are for a reason. The majority of the time, if asked to bet on a highly uncertain thing, the expected value of the probability of the thing being true would fall between 0 and 1 (usually closer to 0).