Because a lot of those leaders come back online again in a few moments. So we wouldn't want to cancel those games.
But, some don't come back at all. What I do after I join a game is check if there's an option to "expel" table creator under his name. That means the leader/creator is offline. Then I check his profile to see if he's been offfline for quite some time now (maybe, more than 3 minutes, which is enough time to restart a computer/router in case of a problem). If he's been offline for a long period of time, I expel him and the game is cancelled, thus, freeing other waiting players to join other tables, instead of waiting for that table to start, which might never happen.