From my experience, I think a lot of it is due to the fact that you can't really surprise yourself. In the conversation in your head, you try to always be the 'winner' of the debuate, and part of that is to try and come up with remarks from your 'opponent' for which you have gotcha replies. Except you know what your 'opponent' is going to say because that's you. In real life, the opponent is someone else and they may surprise you.
Interesting. I tend to think you can surprise yourself. I write stories as a hobby, and my characters often surprise me.
I'd say it's down to two things:
a) Having to imagine a different personality's perspective, and
b) Having some time to think of a better answer than the off-the-cuff one.
Point a tends to happen whenever I make an effort to see the world from someone else's point of view, or make up an imaginary character to have a conversation with.
Point b tends to happen in my own head when I get quiet time - like during a long walk or shower and a thought I'd never imagined before comes up.
It can have some predictive power, too. Imagine, for example, there's an important negotiation coming up. You'll be sitting on one side of the table wanting the things you want. It does help to put yourself in the opposite seat mentally, thinking about what they want to get, and how you'd go about trying to get it. Maybe even trying on for size the best arguments you might make for their position.
True, I didn't want to imply you can't brute force it somehow, but what I was referring to was the general, low-effort, low-stakes conversation we (apparently not) all have in our heads.
