This is similar to college students who study “consulting” as a discipline. Certainly there are important things to learn about consulting strategies. However, if you aren’t a subject matter expert in the field you are consulting, I don’t want to hear it.
For a lot of consulting disciplines, and management consulting in particular, the only domain expertise you require is repeating back to the customer what they told you and finding a way to manipulate and/or present what they've told you such that it supports the conclusion they would like.
This sounds like sarcasm but I'm pretty sure every management consultant would admit that's what they do.
(technical consultants one hopes are actual domain experts, as you say, though I've often found it to be hit or miss even so).
Often the point is that there's no single "they". If an organization is sufficiently dysfunctional that group A refuses hear what group B has to say (and many organizations are), then management getting a summary (filtered!) of group B's recommendations from someone they trust is quite valuable.
I've asked a few friends who are management consultants this. Most say around 25-50% of their job is doing that. They don't view this negatively though because in many cases getting a reputable consulting group to tell you what you already know helps the decision actually get implemented. The rest of their time they spend doing things like collecting information that the client can't/doesn't want to do on their own.
