Because when you come early, not only is the host not ready for you. There are no snacks available for you and you have now all of a sudden inconvenienced the host because they have to serve you, because you decided to be a jackass and come earlier than announced. Be a little late, nobody cares and it makes it easier for you to slip out if its lame. Plus if you’re early, you’ll have to help most likely, which also sucks.