Also, to make it more accurate to what multi-threading does, none of those 300 people can see what the others are doing. And the most reliable ways of sending messages to each other involve taking a nap (though it might be brief, you might wake up in an entirely different body and need to fetch your working memory from your old body or worse, from RAM).

Or you can repeatedly write your message until you can be sure that no one else wrote over it since you started writing it. And the more threads you have, the more likely another one wrote over your message to the point where all threads are spending all of their time trying to coordinate and no time working.