Some poor, rambling presentations might be the result of the speaker actually not understanding the story well enough. A good presenter understands the story one level of detail deeper than what is contained on the slides. If you don't actually know that much about the subject you tend to hold on to the bullets on the slides, start repeating things, go in circles.
Some case examples:
- (Me as a junior analyst in my early days at McKinsey). You just did a big interview survey at the beginning of the project and had to present the current status of the organisation to a room full of executives of the client who obviously knew that picture better than you. Every opportunity was taken to to erode your self confidence. A few weeks and a lot of analysis later, I was full of confidence and could answer every question.
- (Me as a slightly more senior, but still junior, consultant at McKinsey). You were asked to present a piece of research/knowledge that was prepared by someone else in the Firm, you can obviously present the slides, but things get trickier when confronted with questions. Once you had complete a full project in this particular field, you could ace that same presentation, and add your own content for others to present.
- Some start up founders get themselves into trouble when it is time for financial forecasts, go-to-market strategy, product pipelines, etc. If you actually do not master these completely, it will show.
In short, it is hard to "wing" a presentation, you need to know your stuff in order to be really confident. In real life, "study more" is not an option, you are no longer in school, but designing your own slides rather than re-using ones made by others will force you to confront the knowledge gap and show that you did not really understand the meaning of the 3rd bullet point you just read out to yourself when rehearsing.