If you can't see that then you're ignorant of reality.
Speaking of moderating influences, there's something to be said here about maintaining our chill composure.
As for why only moderates. Mostly because the supreme court is a moderating influence. It's not supposed to push us in either direction, or in any direction, it's to hold us back from the worst impulses pulling us in any direction. Make us think long and hard about whether we really want to do this extreme thing, because if we do now we need to take the extra steps of a constitutional amendment or otherwise limit our original plan to a less extreme version.
This may have used to be true, but the Supreme Court's
other job is to interpret what
Congress says. That's not something most people think about, but as Congress has become more do-nothing over the years, the Supreme Court's ability to decide how Congress is to be interpreted has become more important. Consider Obamacare: it twice went up to the Supreme Court, purely because of vagueness in how Congress wrote it. Congress could have superseded the Court at any time by passing a law explaining what they meant. They never did. So that's an example of the Supreme Court effectively doing Congress's job for it.
That's the way it was designed. That is its purpose. That is the only way it can effectively function given the way the government is built. And having a court full of opposing extremists means there is no moderating influence anymore. Simply a continual tug of war.
On one hand, there's a Supreme Court Justice agreeing with you right now. On the other, I disagree that the Supreme Court must necessarily be full of moderates since if you were to take that view, the Supreme Court has
never fulfilled its mandate as it should have. It has almost always been full of people with certain kinds of views: in the late 1800s it was deeply pro-business, for example. Calling this the result of recent partisanship is itself a sort of cherrypicking, since you're looking at two decades of a two hundred year institution. What of the Fuller court, which argued in Plessey v. Ferguson that equal protection clause did not prohibit racial segregation? Or the White and Taft courts, which argued that the 14th Amendment applied to "Legal Persons" like Corporations, and which overturned over the years many labor protections enacted by the states? Then in the 1930s, the Court looked like it did today: a liberal bloc and a conservative bloc (known as the "Four Horsemen", apparently), with a liberal Republican who would side with the latter on economic issues. Then the Warren Court, which made many important decisions, but was almost entirely appointed by FDR (one of the consequences of a 4-term president)? Brown v. The Board of Education, Miranda v. Arizona, the court cases which established the rights to privacy, that struck down mandatory bible reading. I could go on...
My point is that you'd have to ignore the history of the institution to claim that it was designed for moderates, anymore than to claim that the US can't have political parties: you might have had a point, 200 years ago, but it's 2018 now, and fighting to turn back the clock now is equal parts impossible and silly... (It's also remarkably Originalist, ayy).