As you know, JPG compression works by altering the image to produce smaller file size. The reason why you don't see progressively worse quality is because you're using the same compression setting each time.
The algorithm alters the source image just enough to fit into the compression profile - in other words, compressing the result of 50% JPG again at 50% will produce the same image, because the image doesn't need to be altered any more.
You can test this in Photoshop - save a photo out at say 30% quality JPG. Reopen the file you just saved, and go to Save for Web - flip between PNG (uncompressed/original) and JPG 30% - there will be no difference.
Hope this helps.