I promise this isn’t a troll. In your case, it may be that your eyes are having difficulty focusing on nonexistent objects. If they’re blurry, it’s not that your eyes aren’t crossing, but rather that they are out-of-focus. Eyes naturally focus the lenses to bring near or distant objects into clarity, but when I was first doing magic eye images a long time ago, it also took me a while to convince my eyes that they needed to focus on the images.

My guess is that, since the actual images are on the screen at distance A, but your eyes are crossing as if they’re looking at distance B, your eyes are auto-focusing for objects at B, but the images are still actually at A, so they appear out-of-focus.