Interesting question. I guess proofs of algorithm correctness fail less often because:

1. It's easier to empirically test algorithms to weed out the incorrect ones, so there are fewer efforts to prove conjectures of correctness that are actually false.
2. It's easier to formalize what it means for an algorithm to be correct than for a cryptosystem to be secure.

In both respects, proving Friendliness seems even worse than proving security.