In my opinion, people are just poorly educated (or not educated) in presentation making. Or to phase it another way, a school may teach them to use PP the software but doesn't teach them how to put on a good presentation using it.
One key thing people need to learn is: Slides exist to supplement what is being said. If your presentation doesn't work without the slides then you're doing it wrong. Putting exactly what you're saying into your slides verbatim is generally a mistake (although putting key points or a list, and expanding on it vocally can work well).
Generally most people put too much content into single slides as they're worried about having to advance the slideshow mid-point. This means that your point is too expansive, and you should cut your point in half and then cutting your slides in half follows naturally (e.g. instead of doing a yearly projection with summaries of each quarter, just do each quarter individually and sum the year as a whole alone at the end).
I certainly don't think I'm an expert at presentations, I just care slightly more than your average person, and have copied elements from what I consider good deliveries.