Err, often they do. Or more correctly, they often don't show something you think they would if it happened.
Logs show a subset of what has happened. There's no way to prove they are showing everything, so there's no way to use them to prove what did not happen.
Logs show a subset of what has happened. There's no way to prove they are showing everything, so there's no way to use them to prove what did not happen.