While I’m sure the condescension is felt, I think most of us are skeptical that intelligence has much to do with it - it is probably more about ideology, tribalism, persecution complex … as @jpm mentioned above. They probably don’t think of themselves as ignoring mountains of evidence. Perhaps some think of themselves as astutely disregarding mountains of disinformation spread by deceitful conspirators, while bravely holding on to whatever little piece of evidence they deem to have gone their way. Once you have cocooned yourself into that tribe, your filters will thereafter ensure and protect your ideology.
As a separate question, I wonder how uniform flat earthers are as a tribe. The CNN picture featured the flat earther with a cross on his cap. Are the majority of them Christians? of a certain political affiliation? Affluent (or not) social status? What homogeneity [or diversity] could “their tribe” be said to have?