I find physicians like the idea of working for doctors, and hospitals have shrewdly supported the role of 'chief medical officer", while keeping financial controls with the other C level officers. The Children's Hospital in San Diego is famously "Rady's" because the physicians ran it into the ground and the Rafy family bailed it out on condition they accept professional management practices. Not coincidentally, the Rady family also funded the the UCSD Rady School of Management. Also not coincidentally, many health care workers loathe working there, generally attributing this to a belief that management is more interested in the bottom line than patient outcomes, and this leads to a general distrust of Management.

For a professional engineer's license, you have to demonstrate competence and have 4 letters of recommendation from PEs who have either managed you directly, or have overseen your work. So there is not a requirement that you (as a budding prospective PE) may never work for someone who isn't a PE, however it does limit your ability to achieve your own license. Therefore, there is a desire from the younger engineers to seek out PEs to work under.

There isn't in law, but, in most (maybe all?) US jurisdictions, only a lawyer can own a law firm.