Why do tournaments wait for all players to finish before starting new round.

It seems to me that a winner should be announcd in each group as soon as it is certain. This would speed along tournaments. For instance if one player in a group has won all of his games but another two (who are at the bottom) are still playing, it seems that the winner should be announced. Most of the time in tourneys, there are a few players in each group that win a lot pretty quickly, but always one or two slow players who drag everything down (and often dont do so well). And if a new round could start before pointless games had ended, tourneys would be done sooner and more people would enjoy them.

I think the reason you have to wait for all the games to be finished for the new round is because they need to know who won and who lost to generate your new opponent