When I'm in a natural conversation, the list of points I want to make is made up on the fly and is usually quite fluid (changes back and forth as I or the other people talk as a function of what is being said).
That is, talks, even if not already "written", are not the same as natural conversation at all.
I also find that really great impromptu speakers actually do speak more coherently in their natural conversations too. I assume because they're used to pre-editing what they're saying in their heads as they talk.
If natural speech were a modern programming language, prewriting would be analogous to compiling to bytecode. Then you JIT compile it again when you speak. You rarely go directly from source code to machine language, perhaps because there's so much diversity among the "machines" (humans).
Heck, I even "transpile" my talks from time to time. The last time I gave a presentation, my paper was written in English but I had to present it in Korean! Needless to say, that wasted a lot of brain cycles.
I'd also like to note that presentations are very dissimilar to conversations, and some interviews. Conversations are at least a two-directional flow of information (more directions with more people), whereas the bulk of presentations are a one-way flow of information.