Why should only the main ideas appear on a slide for a presentation? enter in 100 words or more, can be a little less

Have you ever watched a dull movie? Have you been bored watching the teacher present slides? Well imagine the teacher presenting every single slide, and each slide has over 1000+ words. Yeah, you would be bored. This is why using the main idea in a presentation is needed; it prevents long slides that could take hours to finish, but it shortens the slide to a point where it can be easily understood without unnecessary detail.