That's not entirely true. Sanderson certainly had a hand in at least part of the prologues. For example, the part of the prologue for TGS where we see random farmer in Saldea - one of Sanderson's standard 'stump speaches' from that tour was a description of the editing process he went through with Harriet for that scene.
edit: and I was amused that the direction Harriet was taking things is almost the exact opposite of what we see these days - basically, more detail and description.
Edited by kcf, 04 May 2012 - 02:22 PM.