This seems like a decent point for accounting but not really for law. Certifying a lawyer is not just about a finite set of things they have to know, it's about fundamentals and the ability to ingest facts and form an argument. Similar to your Haskell point, there are parts of the law that not all lawyers need to know, but are expected to be able to figure out as needed and the certification aims to check for that ability.

(Also, in both cases, the government hires members of the profession to define the standards.)

I'm convinced we could have a bar exam for software engineering, but I'm not at all convinced that we should. It would make my life easier personally, but it would keep a bunch of people who followed less traditional paths out of the industry and I think that would be bad.