That's the silly part of the question. Basically Hume and others are saying that because we don't know every single little piece of information and because there is the possibility, however remote, of an unknown we can't be sure our conclusion is true.

So although we have seen evidence for it time and time again. Even though we have excluded countless of other possibilities Hume would still sit in the back row muttering "there COULD be a different explanation".

Like I said, silly.