Even if they use regulation to limit how many and who is a practitioner, this doesn't give a satisfying answer to why they organize themselves that way and we do not.

Just because anyone can be an engineer, it doesn't follow that we can't form partnerships. I could be wrong, but those doctors and lawyers who are in partnerships don't seem to be using that structure due to lack of employment opportunities under other structures.

Another argument I heard was that partnerships are required in certain fields due to liability concerns so that risk can be contained to individual partners. But this only explains why they must, not why we can't.