That’s not the only reason as pointed out above. Knowing when to crop thumbnails is useful so the thumbnails are actually legible and not just an indiscriminate sliver.
If I uploaded a 100x2000px image, and we capped the height at 500px and resized it the image to fit, you’d be looking at a 25x500px image — which would likely make the image content really tiny and unclear. Using the ratio assures we can detect tall and skinny images that would likely fall into that issue, and then we can crop and ensure that at least part of the image is legible as a preview.