The first thing has to be the unknown. Many religious people will tell you that they know what happens after we die, but since different religions disagree about this I think it's fair to say we don't know for sure. Similarly, we can't be sure how a presentation is going to go. There are so many variables that can go wrong and it only takes one of them to really embarrass the speaker. What if the tech doesn't work even though you tested it in advance? What if there is a jerk in the audience who thinks he is a comedian? Even if we prepare thoroughly and have lots of experience, there are things that can pop up and throw us off.
This leads us to the second thing, the multitude of things we need to concentrate on at the same time to deliver a solid presentation. I tell people who attend my training sessions that presenting is like juggling. You have to focus on what you are saying, how you are saying it, what you look like while you are saying it, and how much time you have left to say it. Throw in presenting in a second language and you might as well be trying to juggle four live squirrels and adding in a chainsaw.
Third is the power dynamic. A lot of people don't like to be in the spotlight. Having everyone's eyes on you while you are trying to manage all the presentation elements is tough; doing it while you are standing up or on a stage adds an extra layer of stress. I know a lot of people that can speak confidently from their seat but when you ask them to stand up and come to the front of the room, they freeze. It's just easier to talk to people when you're on the same level, literally and figuratively.
The fourth thing that makes presentations scary is when they are for high stakes. Sales presentations, job interviews, speech competitions, and presenting in front of your boss are all stressful because of what happens if you mess up. The best case scenario might be that you lose out on some future opportunity but the worst case scenario can be that you lose what you already have, like your job. I think we can all agree on how stressful can be.
Finally, it can especially scary when we don't feel like an expert on our topic. Maybe the topic was assigned to us or we have read a lot of the topic but don't have any personal experience with the topic to fall back on. We don't have a story to tell and have to rely on the information that we found. Impostor syndrome can kick in and really mess with our confidence.
So, yes, giving a presentation can be stressful, but so was driving a car for the first time. Imagine getting behind the wheel and focusing all your attention on the things that could wrong. Your tire could pop on the highway, someone could change lanes and hit you, the brakes could fail. Sure, these are all true, so we are taught to focus on what we can control. Pay attention to what's going on around you, learn basic car maintenance and take it in for regular service, try to obey the rules of the road and things will usually be ok.
A final thing that I tell trainees is that you don't have to be a great presenter, just try not to be a bad presenter. Try to avoid big mistakes and control what you can control. If you do it enough, presenting can become as routine as driving a car. As you increase the size of the audience or increase the stakes, it might feel like learning to drive a bus or even flying a plane, but with training and practice people learn to do these things all the time and it stops being so scary. Hopefully presenting can be the same for you.