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The goal of the justice system is not to be efficient or fast, it is to be just. It is a last resort for complicated cases that can't be easily settled between the parties. You don't want to have every single dispute going through the justice system. That sounds like a dystopian authoritarian future. You want the people in society to self organize and settle their matter privately as much as possible.

It is very much a benefit that it's interpretative and slow. We want it to reflect the culture and people's common sense. We want it to be as fail proof as possible (even if it take ages to come up with all the evidence and arguments). And fail proof here is not to interpret the law in the most pedantic of ways, but in the way that is the most just. The reason for lawyers and courts are exactly the edge cases that are difficult to agree upon.