Hypothesis for why there could be an actual /defense/ for the existence and even utility of flat earthers and similar groups: They act as a continual, low-grade chaos assault on scientific institutions, thus encouraging the institutions to maintain resiliency and not stagnate. The more you simply shut these groups down without tackling their actual arguments, the less benefit you'll receive, and the more likely it is to take on the magnitude of a true threat. On the other hand, too much of this would lead to significant wasted effort.