The 'other' possibility is that hard instances have 'measure zero' in the space of all instances... So when you pick some random problem in the real world, it

The 'other' possibility is that hard instances have 'measure zero' in the space of all instances... So when you pick some random problem in the real world, it ends up being relatively easy to solve with high probability. My feeling is there's a bit of each; the problem space has a bit of unknowable quasi-structure, and also tends to miss the perverse corner cases and counterexamples that we like to think about as mathematicians.