By that logic sport athletes don’t impress you. Movies don’t impress you. Theater doesn’t impress you. Your date won’t impress you. Becoming a parent won’t impress you.

Most things in life take years of preparation.

The issue with preprepared demos is that you can carefully curate the scenario and make choices that you know are likely to show the best outcome, out of a wide range of possibilities. If you know your model (or VC product demo etc) performs poorly under certain conditions, you simply avoid them. This is a reason to be somewhat skeptical about demos.