I would agree with your thoughts. Back when I had my 2010 FX4, I had a number of runs that were 10K on conventional oils and 15-17K on synthetic. With almost every OEM recommending 10K (with the contradictory conditions you note as a safety factor), I am equally surprised that most are not running oil that long.
With that said, it does beg the question of how many "normal" oils can go that long in DI/Turbo engines. I tend to think those type engines would need a "long drain" oil to make it that far and it may be that most are not willing to pay for it OR they are just more comfortable with a more conservative OCI.