Like any profession, one reason for the existence of legal terminology is that it needs to convey things precisely in a way that is not generally necessary outside the discipline. Outsides do not readily understand it because they may not have the legal frame of reference that requires certain nuanced distinctions.

It also has grown organically over a long period of time. U.S. law is a mix of a variety of legal traditions, plus historic precedent within the U.S.

Most U.S. states have legal systems that are largely rooted in English Common Law, except for Louisiana which has its roots in French legal traditions. However, it isn't as simple as that.

For example, water rights and laws in California contain competing premises, one of which is from English Common Law and one of which is not. One of these premises is "First come, first served." The other is that you aren't supposed to take so much water from the river such that you completely deprive people downstream of its use. These are both part of the law in California and they are in direct competition. California alone has a very rich history in development of water rights and laws.

From what I have read, countries that lack the sophisticated legal system such as that we enjoy in the U.S. typically have far bigger problems than what we face here with trying to parse the law.