Law school is a bond, such as a conservatorship bond, a personal representative bond, or a notary bond. That is why little related to the actual practice of law is taught in law school, and why paralegals and document-preparers of all types can practice law in almost all aspects without having a law degree (and why they could do so in all aspects without having a law degree, if it were legal).
The idea of a bond is simple. You pay in order to purchase insurance against your screwing up. For example, as a personal representative, you buy a bond to insure against your stealing the estate assets.
With the law school situation, it is a little more extreme. Not only do you have to pay a sizeable amount of money in order to get your degree, you have to spend three years of your life devoting a sizeable amount of time to obtaining that degree--and typically, three years of your post-college life, which would otherwise be devoted to career. In essence, this ensures that you are more likely to adhere to externally-imposed standards of legal conduct: because if you lose your license to practice, you have also lost any value gained through monetary or chronological investment in your legal education.
Control of lawyers is thus assured. If everyone in the country/world were given the right and the power to practice law, society would come apart. Lawsuits could be much more easily filed. Mass tort claims could be tracked down and consolidated. Any social crusader could turn a formerly-ignored just cause into real trouble for the powerful. The law school time and money bond, and the exclusivity of the ability to obtain that bond, ensures not only that a very few will gain that power, but also that once they gain that power, they will fear losing it, else they have flushed a large portion of their career potential down the toilet, and wasted a lot of time and money in the process.
Law school: 3 years and 50 grand for an insurance policy that never stops charging.