Licensing regimes have a valid purpose: protecting those who have to rely on qualified engineers, when they calculate and advise that something is safe and fit for purpose.
Restricting those who raise concerns that something is not safe or fit for purpose is the opposite.
Not saying you need credentials but it's difficult for outside disciplines to know if somebody knows what they are talking about.
Scary as fuck, especially when one doctor threatened to lock me up if I wouldn't take them.
Sick and disgusting behavior.
However it's true that there are cases where such an expert might be solely relied on, for example they might be brought in by the judge. That happens in family court here, where a judge might appoint a neutral expert to evaluate a child's situation, and indeed there have been complaints that such experts were not sufficiently qualified or regulated. In that case, I would agree that licensing is desirable.