Why don't online video players have an option to buffer the entire video before playing?

I sometimes have an internet connection that runs slower than it can(wireless broadband in an area with less than perfect reception). So I wonder why the online video players, like the one YouTube uses, don't have an option to download the whole video before playing. It is really frustrating to try to watch something, have it run for like 30 seconds, and then stall. usually the best option is to restart the video and let it run and hope the playback doesn't catch up to the buffering. So why don't they have an option like that? Wouldn't they prefer customers to have a smooth playback?