Basically, misjudging the difficulty of designing a superior institution than one that is the result of an evolutionary process that (through trial and error) adapted the particular institution to the rest of society (their norms of behavior) and other institutions.

It is very much related to overconfidence bias ("I really do have a deep understanding of how society works") and hindsight bias ("of course I would have avoided those problems other people had when designing new institutions from scratch, I'm no dummy").

I'm not sure I agree with billswift, but I do think that the people in this thread believe they know much more about how legal systems work than they actually do (based on the comments so far).