I can't speak for all fact checkers, but I'd suspect that sometimes they might be more nuance provider, or in other words: reality checkers.

For example, it would be exaggerated to believe that vitamin D can cure Covid and prevent its spread.

It would also be a bad idea to start high dosing vitamin D unmonitored.

The kind of misunderstanding of the impact and influence of such thing is often an issue when layman hear about medical and other scientific research. So I think a lot of people get worried of some people overblowing things and taking gambles. Science doesn't put its faith in anything, but sometimes layman will put their faith in preliminary results from a few studies.

And another thing is, the science will change over time, and might contradict itself as well, and the layman will again begin to lose faith, because they make it about faith and beliefs, and that's dangerous.