Why is it often so easy to talk to individuals, but when you put those same individuals together it can make people so scared that they go to extreme lengths to avoid it.
1) Evolutionarily, group size matters.
There are many species of birds and mammals that need to maintain a certain flock, pack or group large enough to pack hunt, protect themselves from predators and maintain a high mating population for genetic diversity. If the group gets in an argument or skuffle and separates, then everyone in the group loses. So, how do they avoid separating? With intense attachments and high desires to reconcile. We happen to be one of the species that this was important in during our evolutionary history and therefore we have an build-in system of reward and punishment that keeps us on the track of social conformity so we aren't ostracized.
2) Status is everything for mating. Status determines how desirable we are with the opposite sex and determines how we'll compete with others of the same sex for power and resources (since we don't bash our heads together like rams or inflate a giant red pouch like a frigate bird). Status is also interesting, because it can't really ever be given by an individual. Only groups an give status. Even if a ruler is selected by his progenitor he can only be ruler if there are people to rule. This might be why our subconscious knows that speaking to a group is so important. So much of our status is riding on this situation.
...On the other hand, there weren't many times in our ancestral past that Cro Magnon man got up in front of his peers and gave a Power Point presentation. It could be that our brains simply get overwhelmed when trying to calculate what so many people may be thinking, judging, wanting. Since we don't know how to think about it and the unknown is scary, we wig out...
Other reasons it's good:
3) Sharpens you--the pressures on and every sense is heightened. you may have been tired before, but now it's like you've got Redbull intravenously injected. You're on.
4) They audience can empathize with you. No matter where you are speaking, you have something in common with your audience. Everyone dreads public speaking. Great! Admit you're weaknesses and they are on your side!
5) It creates an obstacle to overcome. If it was easy everyone would do it, but it isn't. It's hard! Think of how great you'll feel after facing your fears! It'll be great!