Why is public speaking so hard?

I have heard that public speaking is the number one fear among adults - I'm not surprised because I am certainly one of those fearful people.
Dan and I, along with another couple, were co-chairs of a big fundraiser last night, called Oscar Night America. It helps support a wonderful charitable organization whose board Dan is a member of. Since the planning for this event started about the same time as Emma was diagnosed with leukemia, I really have not been an active participant in putting on this event. I felt embarrassed that I was called a co-chair even though I had done nothing to help out.
As Dan and I walked into the ballroom where the event was held, a woman on the staff approached us and asked that we go up to the podium right before dinner and thank the sponsors of the event. We said, sure - we'd be happy to --- and I started having stomach pains. The room had about 35 tables with 8 people at each table...quick calculation in my head...I had to say something into a microphone in front of 280 people!
As Dan and I walked through the silent auction room, all I could think about was how I could get out of this... Dan didn't get it at all - he is not a part of that majority of people who fear public speaking. He just thought we could wing it and be fine. This is not how I operate so I made Dan come to a quiet corner so we could decide who was going to say what. I know I was being ridiculous - I only had to say 2 sentences and list 3 sponsors...still...this makes me nauseous.
In the end, I said my few words and it was over.
It was probably a good thing that I didn't know I had to say anything in front of everyone until I walked in the room. I am sure I would have lost sleep over it if I did...