Why don't people realize that the only job of a Supreme Court justice is to decide if a law is or is not Constitutional?

  Supreme courts & judges thereon are the ultimate court of appeal for matters that are presented to it.  The court may decline to hear the matter.  When it does, that's a tacit message that it doesn't see any relevant change it would make in the lower court rulings & it agrees with them.  

  In matters relating to a national constitution, their role is to interpret legislation to determine if it's consistent with the constitution.  If it isn't, the legislation, or its parts  that differ, can be struck down, ruled "of no force or effect."  

  All courts can & do "read into" legislation that's silent or ambiguous on points.  E.g.; The Alberta Human Rights Act.  As written, its enumerated groups to be granted equality didn't include homosexuals.  The Supreme Court of Canada found this made the Act weaker than the constitution which, it its Charter of Rights & Freedoms, used the phrase "in particular" before a series of protected groups.  The S.C.C. ruled those words meant the Charter section wasn't restricted to only those groups, rather, they were mere examples of such protected groups.     

  Katy is wrong about a fetus.  Biology & courts have ruled that it is NOT "human."  In 1988, the S.C.C. struck down Canada's 1969 law on abortion as being offensive to a woman's Charter S. 7 right to "life, liberty & security of the person."  It used biology findings to hold a fetus wasn't "human" unless & until it survived a life birth & took a breath.     

The reason so many assume judges are activists rather than interpreters is because we don't teach the function of such things in schools. Even as far back as FDR's attempts to stack the court along idealogical lines the concept of the judge as impartial was fading. If you don't believe that a lot of people are comfortable with bias in juris prudence just look at half the nation mourning the loss of RBG and talking about "what great things she did for women". Judges aren't supposed to do great things for one group or another, they're just supposed to interpret what's already on the page. 

Virtually all laws are interpreted to be applicable or not applicable under certain conditions (such as abortion). The interpretation is not "black and white" and it is not included in the Constitution. When the Constitution was written abortions may have been uncommon but they did occur and they were not against the law

Your lack of knowledge should be embarrassing. 

No, that is not the “only” job. The Constitution is not a suicide pact. The Supreme Court sometimes has to make rulings that are clearly against the Constitution, because otherwise the country would be destroyed. See, for example, rulings about the civil war, and the 9/11 response. 

Besides that, the Court makes rulings on many subjects where Constitutional issues are not involved.

The Constitution is irrelevant to Democrats.  All that matters is the lib agenda, and how they can bend the law of the land to achieve their goals.