Well our eyes don't exactly stay in that "defualt" position all the time, whenever you use your eyes at all and focus on anything, they look at the same point, and to do that they have to have an angular difference because of those two inches they are apart. If they didn't have an any angular difference and our lines of vision were always parallel I don't think the brain could correctly process the two images and put them together into one picture with depth. So doing that with game images wouldn't work right
Although I'm not expert on how eyes work, so I don't think I should be arguing too much about it.
Hell, I ain't no anatomist. Just look at the geometry and optics of the eyes and you could understand.
You are almost correct. It's better to take the pictures without rotation because that is a default setting for the pictures, just like looking straight on with our eyes parallel is default. By rotating the camera angle, that would be the equivalent of splitting and rotating your entire environment around in opposite directions, which is why camera rotation looks wonky.
WARNING: the next paragraph is very long and has useless procedures to help you understand, but probably won't help, skip to the next paragraph after this for a less detailed way of thinking about it.
Imagine a plane perpendicular to the line between your eye and the object you are looking at. Do the same for the other eye. Be default, these planes are parallel to each other. When looking at a picture on a screen with one eye, and the plane is parallel to the screen, you are looking right at the picture. If you were to open your other eye, and close the first, without focusing the second eye on anything else, your eye would be looking at a spot about 2 inches away from the spot your right eye was looking at. Now, as you rotate your eyes inward, your screen begins to split, and you can now see a screen on the left, and a screen on the right, and they are both seemingly angled inward. Now, lets put two pictures, side by side, of an object from two perspectives without rotation. Our eyes are at default, and for simplicity's sake, the left eye is looking at the left picture, and the right eye is looking at the right picture. If we cross our eyes, the two images creep toward each other until they overlap. Now, our brains have a special trick, commonly dubbed X-ray vision. We can see behind objects because of the perspective of each eye, though the image is discolored because our brain combines the images of close and far off objects, and thus combines the colors together. This also happens with our two pictures. Once our images overlap, since they are of the same object, and therefore have the same general look, the colors don't merge, and we get a uniform looking object (test this out by applying the cross eyed skill to those Find the Difference puzzle pictures. Just cross your eyes and look for something wonky looking, there is your difference). This overlap is what makes something look 3D, and in our world, we are used to have to look at a single object, not an object split and rotated magically. Every line of default mode has to be parallel.
In real life, when looking at a single object, if one eye's line of sight is parallel with a straight edge on that object, then the other eye cannot be parallel with that same parallel line on that object, and there is a certain difference in angle between the parallel line in the first picture, and the non parallel line in the second picture. Let's call this our viewing angle. So, in blockland, if you want to replicate IRL 3D, you would want to not rotate, because your brain already accounts for the viewing angle by rotating (slightly) the images seen in your brain. Therefore, if you were to rotate the image ingame, you would be adding that angle to our viewing angle, distorting the image. It can still look 3D, but the dimensions would look messed up. Rotating it in would squish the dimensions, and rotating out would expand (if my half asleep brain can think out the geometry correctly).
The reason I'm responding with all of this is because I did a lot of messing around with this kind of stuff during the last days of school this past year - for example, on a white piece of paper, draw a dot, then draw another dot about two inches away, then cross your eyes. The dot you now see in the center of your vision looks 3D. Or hold two similarly looking pencils next to each other, and cross your eyes until you see the pencil in the middle. Experiment with the distance between the pencils and how that affects the 3Dness of the resulting third pencil.
