Why? Because they don’t understand that the goal is to engage the viewer’s perspective.
It’s the same reason fashion photographers blow clothing shots by not including the model’s face, thereby ensuring that the opposite sex will not engage with the shot and so immediately lose half the potential buyers; it’s the same reason many photographers forget to tell the model / subject to take the sunglasses off (if the eyes are the windows to the soul, sunglasses are surely the window-shades — the immediate result, again, is a loss of engagement); it’s the same reason that for the (wholly imaginary and vaguely ridiculous) sake of “art” that photographers pose people in graceless and awkward positions, and/or with disinterested or unfriendly looks on their faces, thereby almost certainly ensuring an uncomfortable, disinterested, or unfriendly viewing experience. There’s more like this, but they’re really all the same error at the root:
These problems occur because the wrong attempts to please are in place: the photographer’s, the clothing manufacturer’s — basically everyone but the viewer(s) of the photograph.
If you want to be a truly great photographer rather than just a pretender, you put yourself in the viewer’s place, and no one else’s. If you please the viewer, the photo will grab them; if you don’t, they’ll barely see it, and they surely won’t be likely to remember it fondly or in any way that makes them interested in a product. If you actively displease them, well, you’d better hope you pleased yourself, because otherwise you just completely wasted everyone’s time.