[charleskinbote]

I've asked myself the same. I think of it this way: we can imagine that, in the set of all possible solutions to a particular problem, a subset of those are too hard for humans because they are solved with systems too large for a human to have designed in a reasonable amount of time, or the systems themselves contain pieces that are composed in some previously unrealized way. Of that subset, there may be solutions whose pieces and compositions are well-understood. In these cases (perhaps not exclusively), it might be possible for us to begin understanding any rationale.

edit: after noticing this other hacker news article (https://news.ycombinator.com/item?id=21107706), I wanted to add that this line of thinking is applicable to understanding programs and proofs written by humans as well. Programs and proofs can be well-understood when their pieces, and the way those pieces compose, are well-understood. When the pieces, e.g. lemmata in a proof, are large or hard to decompose, the proof (i.e. the solution to a problem) is harder to verify and understand.