There are ways to make mathematical estimates of the number of stars that exist. We tend to believe something like that because scientists deal in facts, and there's no way for the average person to disprove it. Wet paint is another thing. It's something we can touch to see for ourselves. Man is a curious animal.
I'd say just human nature. Also it's easier to check wet paint than to count 4 billion stars.
It is easier to believe the things we cannot contemplate, but so tempting to touch wet paint for your own test he he.
I think it's because they cannot count the stars and therefore check what you are saying, but with paint, they can validate that.