Because they're bound to a body? Sasuke could only do that because the Bijuu were in their physical forms. Hence why he could channel the chakra through his own CT. Being able to do that through Naruto doesn't make any sense. It's like saying Naruto and the alliance could take the Bijuu's chakra out of Obito without Naruto needing the other Bijuu's chakra to do it, when the manga confirmed that he could do that only because of the Bijuu's chakra in him. Sasuke would need his own Bijuu chakra fragments to take them out of Naruto.
You're not getting the point.
I said at first that he used preta. You responded saying it was the outer path. I then accepted this possibility.
The outer path can be channelled through gedo mazo or directly from the user.
Pain used the outer path in the bijuu extraction process. [extraction from jinchuriki]
Sasuke supposedly did the same with his Susanoo. [extraction from bijuu]
My point is that the outer path gives the user the ability to extract bijuu chakra, either from the bijuu directly, or from jinchurikis.
Sasuke did what akatsuki did with gedo mazo using his Susanoo. He should therefore be able to do the same to Naruto who is a jinchuriki.
Do you get my point?