Look and feel. At least, that's why I go along with the convention
Since the real feel of the depth is based on the depth at the waist/shoulder against your chest, tapering head to tail is sort of the same thing as a Manzer wedge, just in the other axis. It also just looks more natural, with the whole thing being generally triangular when viewed from the front, with the wide lower bout, skinnier shoulders, and skinnier yet headstock. Another reason I like triangular-ish headstocks, to continue the shape all the way to the peak
There is one builder I know of, Stephen Sheriff/Edwinson Guitars, who does coplanar top/back, and says it sounds better. Does seem believable, than the plates could activate eachother more easily pushing the air directly up and down. But probably not that big a difference, and isn't worth the look/feel sacrifice for me to bother trying it. I love Manzer wedge.