Many years ago it was decided that we handle 2d images as 3d images with a single slice. While not correct as the pixel spacing in z-direction is completely made up, it reduced some code and edge cases and was found to be an okay-ish simplification. On the other hand it rarely leads to issues that were found to be worth having them as the reduced code and edge cases gave way more benefits in comparison. One reason for this discussion in the first place was the segmentation btw as additional 2d cases regularily broke even more code than all the 4d cases. So eventually it was said, that it will be kind of guaranteed that loaded images will be presented at least as 3d images. It is still possible to generate 2d images in code and that's why there still should be checks if an image has less than 3 dimensions.