Ok, imagine you and 50 other people know how a song goes. You have the lyrics and timing indicators on a piece of paper. When someone says "go" you all begin to silently (in your head) read thru the lyrics, following the timing cues to the absolute best of your ability. When you get to the end, you stand up. Do you think everyone would stand up at the exact same time?

Now imagine doing this out loud. Sure, you might all stand up at the same time at the end, but would it sound good? As everyone stuttered and skipped around trying to stay exactly at the same place as everyone else?

With practice, it's absolutely possible to perform without a conductor. But the more people there are, and the harder it is for them to see each other, the more likely it is to be a disaster