Because the Constitution is not a clear, unambiguous codex the application of which requires no judgment. If it were, then stare decisis would be beside the point because it would be obvious how it applied in every single case.
But that is not the Constitution we have. The one we have, like almost every law we have, contains ambiguities and must be applied in circumstances the authors could not have imagined. Doing that requires judgment, and it is the role of stare decisis to give respect to prior exercises of judgment so that the Constitution does not become a football and application of the law a matter of the personal whims of the judge who happens to apply it.
Respecting judgment calls made by earlier judges in ambiguous or ambivalent cases, even if one might personally exercise one’s judgment in a different way, subserves very important interests that lie at the root of any common law system that has a claim to being the rule of law, not merely one of personality.
Considering that most "ambiguities" are the result of semantic distortions produced by attorneys, when most of the Constitution is actually pretty clear to people who do not engage in such RCH splitting and twisting to push their agenda (or that of their clients),it isn't hard to see how we have ended up with the mess we have.
There are, however, logical distortions which exist, even within the semantically distorted framework of the Courts.
In Miller, the judges allowed the lower court ruling that Miller was guilty of a violation of the NFA to stand, despite their interpretation of the 2nd being that it preserved only the right to keep and bear weapons useful in a military context.
Short barreled shotguns had been in use as trench weapons by the US military in the First World War, and, by that train of logic, should have been included in those weapons left uninfringed, along with any weapon used as a weapon of war.
The lack of knowledge by the justices that shotguns were, in fact, a useful militia weapon (still deployed today, were in Vietnam, and in conflicts prior), led to a ruling that would be erroneous in even that limited context.
A broader interpretation of the RKBA (the correct one) would have simply thrown the NFA out as an unconstitutional infringement, and successive legislation limiting the Rights of the People would have fallen to the gavel as well, if even considered by Congress.
Reading the writings of the Founders on the Standing Army, the role of citizens in keeping that Army "well regulated", would indicate that that Right of the People should be broadly interpreted, only limited through restrictions placed on those who have committed a serious crime (through due process of having been found guilty of that crime).
