These two images are very similar. Why doesn't SSIM reflect that?
clearly, these are very similar images


Both I and your arbitrarily picked measure for similarity would like to heartily contradict you, there.

It was you who picked SSIM, Structural SIMilarity, as measure; if that measure doesn't describe your own idea of similarity well enough, well, you might want to define what similarity is and come up with  a different measure. 

Here, specifically: SSIM is especially designed to respect how focused, sharp, high-contrast images are. That's the whole point of it: although a low-pass version of your two images would indeed be very similar, the SSIM, meant as perceptive quality index, should identify that the second one is very different.

So, you picked pretty much the worst metric for your idea of similarity, and now you're "complaining" that it does what it's designed to do!