I know why they crack at all, but I'm actually surprised that almost boiling water doesn't cracked them best.

The cracking is due to the fact that almost everything expands as it gets warmer, and if one part expands (or shrinks) more than other parts the stress forces can cause the connecting areas to snap and disconnect.

You get the same effect and can shatter glasses and mugs by very rapidly cooling or heating them (think hot tea in a cold glass, or ice cold drinks in a glass from a hot dishwasher).

My best guess on it NOT cracking in super hot water would be that it melts the surface quicker than the layer of warmed (but still frozen) ice can get thick enough to have enough stress built up to shatter free.

It's also possible the latent heat absorbed by melting off that surface is acting to cool the transition a bit, so the faster it melts the (effectively) better the cube is insulated, as sort of an ablative thermal stress guard, but I would expect that effect to be swallowed up by the vastly greater thermal difference without much notice.