It could be because we have an inherent desire to prove or disprove what we have been told. In the case of the billion stars, most of us don't have the ability to count them, and when we look (there is the desire to prove/disprove,) we see what appears to be proof that the statement is correct. However, in the case of the wet paint, we DO have the ability to see if the statement is correct, hence the compulsion to check it for ourselves.
The possibility of there being a billion stars in the sky is very believable. Ask someone not to touch the wet paint is a questionable thing, it could actually be dry! And how many stars are left after some of them becoming shooting stars? I would be one of those that touches the paint!! lol!