The idea, at least, was to keep them as independent of politics as possible.
AFAIK, most of the government does not have limited terms. The President was limited to two terms by a fairly recent constitutional amendment. I don't know of any other successful attempts to impose term limits on other politicians.
In practice, the Supreme Court is influenced by politics in a similar way to the other branches of government, but they are the slowest to respond to changes in the political landscape. Conservatives have had much greater influence over the other two branches of government for 10-20 years now, but we're just starting to make our presence felt on the Supreme Court. If we hold power for another 10-20 years, the Supreme Court may be a fairly conservative institution.
Now, it's a matter of debate whether it's time for a change in how Supreme Court justices are handled. It may look like a good idea now, but what could happen 50 years from now? What if we get our conservative court, and Liberals start to take back some power? We might be glad to have our Conservative court to moderate their legislation.