The language isn't particularly complex, it is very precise. Though legal documents do use words that are particular to the field.
The reason such precision is required is that our legal system is "adversarial." That is to say, if you get two sides arguing as best they can, there is a good chance the truth will come out.
The procedures are nothing more than the "rules of the game." If you looked at the rule book for football, you'll see very complex procedures as well.
However, what you may be really saying is why can't it just be "common sense?" The answer, of course, is that "common sense" is more fallible than highly organized procedures.
I'm thinking about the old Workers' Compensation laws in California, before they were revised by the legislature last year. I read that the laws and procedures were so complex that most workers had to hire a lawyer just to file a workers compensation claim for a workplace injury and pursue the claim to completion. If the average person needs a lawyer just to file a claim for a workplace injury, then the laws and procedures are too complex. They're written that way by lawyers to generate more work for themselves and other lawyers. Just follow the money, Jake.