I think that's because in LF, the shades' shapes are dependent not only on the lens but also on the format, on the amount of tilts and shifts, on the focusing distance, and so on. So as B.S.Kumar said, hoods must be adjusted for each image.
I don't think this is necessary. Myself, I prefer to keep the hood front parallel to the film plane. Hoods I consider the most convenient are those that attach to the camera, not to the lens. That way, the lens can be tilted while the hood remains in the same position and needs no readjustment.
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