This is true for most state-licensed professionals like doctors and accountants. There are two main reasons for this.
The first is that a non-licensed person could easily bypass the license rules by affiliating with a licensed person in name only and doing all the work requiring a license.
The second is that licensed professionals have ethical and professional duties to their clients that they are taught as part of their licensing eductation. A non-licensed person may not know these duties, or be aware of their rationales, and may try to influence the licensed person to violate them.
I don't believe this is true for doctors, although, I could be wrong but I think HMO's get around the restriction. The bottom line is that lawyers are a protected guild with rules like the one mentioned above that artificially inflate the value of their services.
The first is that a non-licensed person could easily bypass the license rules by affiliating with a licensed person in name only and doing all the work requiring a license.
The second is that licensed professionals have ethical and professional duties to their clients that they are taught as part of their licensing eductation. A non-licensed person may not know these duties, or be aware of their rationales, and may try to influence the licensed person to violate them.