This reasoning is problematic because it ignores the possibility that the truth or falsity of something might just not be known yet. Or that something may be incredibly difficult or impossible to know. This argument shifts the burden of proof to the opposite position instead of providing evidence to support their own position.
“The existence of life on other planets has never been proven, therefore it is reasonable to conclude that such life does not exist.”
“UFOs must be aliens, because there are so many cases where scientists dont’ know what happened and can’t prove otherwise.”