I can't understand why people include GitHub as a deploy time dependency. Deploying should be about pushing an already built package to the destination server(s

I can't understand why people include GitHub as a deploy time dependency. Deploying should be about pushing an already built package to the destination server(s). The idea that you push the source code then build it on the server makes no sense to me. If I was github I would forbid (and punish) this kind of behavior.