That actually seems to be quite an important property in highlighting what a cool effect it is. Someone else here posted a link to a similar project they'd done [1]. If you follow the link and press T to switch on the noise, and then A to start the box moving, you'll see how obvious it is and the effect just isn't as cool. However if you then press B to switch on the background movement the box suddenly become harder to see until you correctly focus your eyes

1 - https://moefh.github.io/stereogram/index.html