In a political system that places such a high value on electing officials to office, it may seem strange that Supreme Court justices frequently serve for the remainder of their lives. It's certainly a contentious topic in some legal and political circles, but the reason why the Supreme Court is a lifetime appointment stems from the Constitution, or at least, from a general interpretation of it.
The Supreme Court itself is established by Article III of the U.S. Constitution, which says that "judicial Power of the United States, shall be vested in one supreme Court... The Judges, both of the supreme and inferior Courts, shall hold their Offices during good Behavior..." The clause which mentions that justices "shall hold their Offices during good Behavior" has long been interpreted to mean that they will continue to serve for the rest of their lives, unless they retire or are impeached.
One argument in favor of lifetime appointments insists that, because the justices know they will have a well-paying job for the rest of their lives, they have less incentive to be driven by political goals. In other words, they are more likely to exist in a realm that is above the daily political debate, removed from self-interest, and only be focused on interpreting the quandaries before them.
But while lifetime appointments have been in place for literally hundreds of years, the relative wisdom of allowing justices to serve for their entire life has been debated since the Constitution was written. One person to express concern about this practice was Thomas Jefferson, who hinted at the potential negative side effects of allowing people to have a position for (possibly) decades.
"Our judges are as honest as other men, and not more so," Jefferson wrote to William Jarvis in a letter about judicial review. "They have, with others, the same passions for party, for power, and the privilege of their corps ... Their power [is] the more dangerous as they are in office for life, and not responsible, as the other functionaries are, to the elective control."
But you don't need to be Jefferson to have reservations about whether lifetime appointments to the judiciary have a proper place within a democracy. A Reuters/Ipsos opinion poll from 2015 found that the vast majority of Americans would prefer that there be a term limit on Supreme Court appointees. Per the poll, 66 percent of Americans think that 10 years on the court would suffice for each judge.
This debate has been reignited amidst the chaos surrounding the nomination of Judge Brett Kavanaugh and the allegations of sexual misconduct against him. (Kavanaugh has repeatedly denied all the accusations.) If he is confirmed to the Supreme Court, it's possible that he could be removed by impeachment, but that would be an arduous process, and certainly no guarantee. (Just look at what happens when presidents are impeached, after all.)
In theory, if his opponents knew that he would only serve on the court only for a limited time, it's possible that they would be less perturbed by his potential confirmation. There would be an end in sight, in other words.
For now, however, the system remains as is. Changing it could require a constitutional amendment, which, while not impossible, certainly doesn't seem to be a top priority in the current Congress.
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