because it looks flat if you don't, sketch up creates 3d environments but it doesn't give it texture, or line weight or anything other than bland generic shading. I mean just compare these two
can you see what I mean?
the sketch up one, while technically impressive and realistic, it is flat, boring completely lifeless, while the second one has a purpose, character, the lines aren't dead same with lines they have variety, the shading is made specifically to convey the scene in question.
now this is all just my opinion, but I believe sketch up should be used as a perspective and background building tool, not a replacement for the actual backgrounds, using it for making complex and detailed "Sketches" that the artist later should draw over and give weight, shadow, fading and textures or simply a reference to how to keep the place looking consistent.
but again that's just me