An institution that's hiring fresh lawyers or fresh doctors can piggyback on the extensive, rigorous and difficult evaluation (which costs a lot of time and effort both for the candidate and the evaluator) during the bar qualification, medical licensing examination, etc. For example, if a potential doctor is a board certified proctologist(colorectal surgeon), then you can just assume that they will be able to do proctology to a high standard, you don't need to (and likely even can't) verify that aspect of the candidate. (Though employment of doctors itself is quite different from how hiring normal employees work, they're often more like independent contractors or running their own practices)
If an institution is hiring a fresh software developer, then a CS diploma does not even reliably guarantee that they know the basics of software engineering (e.g. see yesterday's discussion in HN https://news.ycombinator.com/item?id=24740765 ) - so you have to do all the evaluation yourself, and neither you nor the candidate would be willing to spend as much time and effort on that evaluation as a lawyer or doctor does, so it definitely is trickier to evaluate since you don't (can't) have enough information.