The way in which one attorney explained it to me is that the Constitution can almost always be used to settle procedural issues but not substantive ones. That is if we want to know how old do you have to be to run for president, that's defined in the Constitution.

But if you want to know what is the precise difference between fair use and copyright violation or where the limits of the 14th amendment are or what's the difference between a tax and penalty or other substantive questions, the Constitution alone often does not speak to those issues sufficiently.

The Court has to interpret whether certain textual provisions apply to certain substantive situations which results in case law. The other issue is that not every question raised today was contemplated in 1789. There's nothing in the Constitution that states that "everything herein must be interpreted as WE understand it in 1789". And that's a good thing too as some of the Founders had pretty problematic viewpoints. In any event the Courts under Article III must interpret the law and apply it to cases as they arise. Imperfect but so is any system.