It's incredibly important NEVER to reveal information visually before you get to talking about it. With scrolling, you'll be showing bits of the "next topic/points" at the bottom of the screen (unless you're incredibly careful) and people will be paying attention to that and guessing what comes next rather than paying attention to the discussion. (Don't know why people do, they just do. People are curious and easily distracted.)

Therefore, you need a fixed canvas to flip through, for total control. Hence, fixed slides rather than scrolling.

This is also why when slides have multiple main bullet points, it's very common to only reveal them one bullet at a time. This ensures you have the audience's attention as you discuss each one, and their mind isn't distracted by thinking about the next.

So that's the main reason -- to carefully and precisely control the flow of information, to sync talking/discussion with the visual.

Secondarily, slides lend themselves to attractive layout of images etc. for maximum effect of storytelling and emotional effect. Of course, for a highly technical presentation, that may be irrelevant. But a lot of talks do include a storytelling/emotional aspect where the dramatic framing of a single photo is important, which you need a slide with a fixed layout to do.