Yes, I thought the same thing, but not all VESA mounts are the same, so I created a flat plate to give rigidity to the mount when the unit is hanging on it. Some have pretty thin 'X' shapes, and given that the bottom holes are not bolted I thought it better to spread the weight over the whole mount.
If you have a simple post you get lateral force so the top pillars have to be reasonable wide, but the bottom ones would simply be pressing back.
The mount only adds 6mm to the depth (or reduces the fan space by 6mm if you like), so I figured it was acceptable.
Perhaps a boundary only, rather than a full plate, might work? That would keep everything rigid, require less plastic, and would print quicker

The plate itself was adapted from one I created with internally embedded Neodymium magnets, as I was considering using the plate as a magnetic mount for a battery pack, which I'll test once the one I ordered arrives. This would also have to be kept away from the fan, so a plate will be required for that.
What do you think? I'm very open to ideas and suggestions.