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That's something I've never understood about myself: I'm terrified to ask a passerby for the time of day, or ask a prospective love interest out on a date, or even call to order a pizza, but I have absolutely no problem standing in front of a room of strangers and speak for hours and hours. My suspicion is that it's a power dynamic issue: when I need to ask something of someone, I'm at a disadvantage, but when a group of people have already been convinced to gather in a room with little choice but to listen to me, I don't need to be afraid.
Another hypo: it is a confidence issue because you know you are an expert of what you speak about and the audience is interested and how good a presenter you does not matter so much. So you are full of confidence. You know you are giving them a good product, you know they want to buy this product, maybe the packaging is not so good but that is okay.
For the other situations, you don't know the product is good and you don't know they want to buy.