People selling runs, why not talking about it ?

[Niwashi]

I think you are missing the concept of the reason WHY people pay for runs in the first place.

That's entirely possible, as I've never paid for a run myself, but the reason I see most often mentioned (as well as the only reason I can envision ever having), is because of the difficulty in finding 7 other players to do the run with you.

No matter how much you yourself might learn in a failed DF run, if your next DF is with 7 other players who haven't learned it, you won't progress much further. DF is with different players every time, so you can't really get learning progression in the overall group like you can with a static. For any content difficult enough that you need 7 other players who know it, and if you yourself don't know 7 other such players, it makes some sense to seek out groups who know it and will run it with you, just because that's your best chance at being able to do the entire run all the way through.

Assuming that's the most common reason (as it's certainly one of the most common reasons) to pay for a run, the only way the person paying gets what they paid for is if they get to participate in the entire run. That's what they're paying for the chance to do.