This is not how things ever work in practice in representative democracy. The world is too complex, and the many overlapping sets of political groups in a country/provice/city have different takes on what the policy should be, and more importantly, each group have different tolerances for what they will accept.
Because everyone has different principles by which they evaluate the world, most laws don't actually care about principles. They are simply arbitrary lines in the sand drawn by the legislature in a bid to satisfy (or not dissatisfy) as many groups as possible. Sometimes, some vague sounding principles are attached to the laws, but its always impossible for someone else to start with the same principles and derive the exact same law from them.
Constitutions on the other hand seem simple and often have simple sounding principles in them. The reason is that constitutions specify what the State institutions can and cannot do. The State is a relatively simple system compared to the world, so constitutions seem simple. Laws on the other hand specify what everyone else must or must not do, and they must deal with messy reality.
This is not just unhelpful (and overly cynical), but it is untrue.
Courts follow the law, but they also make determinations all the time based on the underlying principles when the law itself is not clear.
Law school itself is largely about learning all the relevant principles at work. (Along with lots of memorization of cases demonstrating which principle won where.)
I understand you're trying to take a realist or pragmatic approach, but you seem to have gone way too far in that direction.
Because everyone has different principles by which they evaluate the world, most laws don't actually care about principles. They are simply arbitrary lines in the sand drawn by the legislature in a bid to satisfy (or not dissatisfy) as many groups as possible. Sometimes, some vague sounding principles are attached to the laws, but its always impossible for someone else to start with the same principles and derive the exact same law from them.
Constitutions on the other hand seem simple and often have simple sounding principles in them. The reason is that constitutions specify what the State institutions can and cannot do. The State is a relatively simple system compared to the world, so constitutions seem simple. Laws on the other hand specify what everyone else must or must not do, and they must deal with messy reality.