Why am I still anxious about things that I know won't happen?

because in the back of your mind, you feel that there's a possibility that it could happen. Whenever i get like that, I ask myself, "okay, but has it happened yet?" usually, answer is no. That helps me a lot.

You might be experiencing some anxiety, don't over think it, if you begin to feel this way, distract it with something less scary.

It may be that you have an overcautious nature that sends you these signals. It may be better to be safe than sorry, but there is also benefit to realizing that most of your fears are unfounded. Oftentimes getting enough sleep can help.

Because as anxiety is a state of mind it's defined as a fear of something that is in the clear and present danger or something that is a possibility that you fear of happening which sends you into a pit of worry and dear of something that could may never happen. In your case of something that you are for sure won't happen it's that part of the definition where even though you know your brain still allows you to be scared of just the thought, people who have anxiety typically run through scenarios in their heads allowing them to visualize it and become even more scared of something that you clearly know will never happen.