If a speaker should specifically train him/herself for a talk, the talk is not worth listening.
I've given close to 300 sessions at about 100 events over the last 10 years.[1][2] The top three things you can do are practice, practice, practice. There are nuances to the material that you don't even think about at this point. Saying it out loud makes you think about them.
There are jerks in the audience who will say "do you know about X?" which is semi-related but not completely.. you need to respond and either a) connect back to the topic or b) shut it down and redirect back to the topic.
When someone asks "that's great but I need to worry about B instead of A, what do I do?" you need to either punt to something like "catch me after" or "Oh, there's a setting/plugin/tool for that!"
Finally, there are transitions and a flow to the talk that you don't know about until you try it. And sometimes you'll need to look up things - about tools or even your projects - to refresh your memory.
Admittedly, I go lighter on practice because I was a theatre geek (only 3-4x out loud before the first time) but it is still the most important step.
In a different sphere, there are people who are naturally very funny but fail as professional comedians. There are people who are not naturally witty who do make good comedians, due to approaching it like a challenge of writing and performance.
I don't think you can simply say that people who are unable to wing it aren't worth listening to. Having said that, I do admire people that can wing it.