@Soulfox -- Because a lot of time a person might be brilliant at coming up with a profitable idea but is horrible at running a business. The "business side" of things can get very complicated in a hurry.
Let's talk about the lawyer you mentioned, for example. She may be a brilliant lawyer, but you don't really learn how to run a law practice well in law school. That is a skill learned by people who earn degrees in business administration or not. Lawyers learn how to sue people. That lawyer might be smart when it comes to her profession, but dumb when it comes to building a business and making it work.