It's a tragedy of the commons problem. For any individual author of cryptolocker malware, your incentive is to just not bother writing an unlock mechanism. It's easier to just overwrite everything from /dev/urandom.
If a sufficiently large proportion of cryptolocker malware didn't actually have an unlock mechanism, it would become apparent quite quickly. We'd see reports on places like HN and SO, ultimately trickling out into the popular press. Only the most naive victims would ever bother paying the ransom, because it would be common knowledge that it doesn't work.
In the long run, ransomware is only successful if paying the ransom usually works. There may be an element of collusion amongst organised criminals, or simply a sense of personal pride by the authors.