Disclaimer: I'm neither a JD nor a philosopher of law. But I don't think that will matter for the points I want to make.
My guess is that very few laws were written by people with training in philosophy of law, and not too many more by people with training in philosophy of any sort. But I'm not sure that should worry us.
The first reason is that laws in many countries—including the United States, where I live—are part of the democratic process. They neither are nor pretend to be examples of perfect justice or perfect efficiency. They're the result of various kinds of compromise and without a dictator, that's how it has to be.
Furthermore, in democratic systems, the question of what justice really demands isn't the only one we think should guide lawmaking. People can disagree about what's just. In democratic systems, the will of the voters counts. Suppose a legislative aide has been charged with drafting a law. Suppose she is well-schooled in legal philosophy. And suppose that the statute she's meant to draft doesn't fit her considered view of what the law should be. If it's something the voters clearly support, her contrary opinion won't matter. And unless the law is deeply and manifestly unjust, it shouldn't matter. We live in a democracy, not a philosophracy, to coin an ugly term. If the philosopher wants her view to prevail, she should run for office, or try to persuade voters by becoming a pundit or commentator. (I'd add: even then, it's a mistake to think that philosophers have more insight into what's just than everyone else does.)
Finally, suppose a proposed law seems fair and reasonable. Turning the proposal into a statute that will function efficiently in practice is a specialized craft. It calls for a practical understanding of how the courts, government bureaucracies, businesses, etc. actually work. There's no reason to think that training in philosophy of law makes someone an expert on any of that.