there is little to no room for being "just"
... The Supreme Court "Justices", in particular, like to imagine themselves as "calling balls and strikes", regardless of whether the resulting judgment matches anybody's notion of justice.
The reason they have to do follow the law without imposing their arbitrary whims on the people that come before them, is that the law is BY DESIGN written by people who have to stand for reelection after they pass the laws (yes, there's the issue of whether our representatives actually represent us, and whether outside factors such as big money unduly influence elections, but it's not particularly relevant to the principle that's being questioned here).
There is one dimension of justice in everybody following the same law, but only one of many.
The idea of laws applying equally to everyone is a HUGE DEAL. It's the practical application of the whole "all men are created equal" thing.
Judges following their own notion of "just," and disregarding the law in cases where they felt it was unjust, would cause a lot of decisions that you would certainly find horrendous. As it is, judges face very little accountability. The supreme court justices are appointed "in good behavior," which is usually interpreted as "lifetime appointment, with the option to impeach them if they start acting completely horrible." So there's a very good reason why their job descriptions leave very little wiggle room to do whatever they feel like. Yes, the supreme court frequently makes decisions that I find appalling, but at least they have to back it up based on law and precedent. In the system you're yearning for, they wouldn't even have to do that, if they felt that "justice" (whatever they felt like that meant that day) demanded it.
I would propose that the next time a "Justice" declares himself to be "calling balls and strikes", that we rename his title to "Umpire", as "justice" is orthogonal to his self-defined description.
Call them "Umpire" if you want, but under your proposed system, we'd have another title for them - monarch.
I can't believe I have to spell this out, since it's high-school-civics-level stuff