It's a long way to the chemist, etc...
But space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is.
Humans have been emitting radio waves for about 100 years or so. That means that any intelligent life more than 100 light years away (which is next door in galactic terms) simply won't know we exist, since the signal won't have reached them yet.
Contrary to what the prof thinks, there could be loads of little green men out there, all whizzing about in their flying saucers, and they haven't visited earth yet, since they just haven't found it yet, or when they last looked, it wasn't doing anything interesting.