I recall someone saying something about news, and how when you specialize in something, you read an article in the news and go "This is complete nonsense, it's all wrong, and disgustingly so" but then when you're done with that article you move on to the rest that you're not an expert in and for some reason forget about how that first article was wrong and assume the rest is completely accurate. A retraction has that effect on readers. They go "Oh, that was wrong, huh? Wild" and then move on to the next article as if the news has never been and is never wrong, and the same with Snopes. They've been caught being wrong before, but somehow people just move on to the next 'fact check' and forget that Snopes has ever been wrong.
Snopes just doesn't correct itself - like newspapers are loathe to retract except in tiny back pages to avoid liability - because they want to control the narrative as much as possible, and that includes lying and manipulating the truth.