Power
Lifetime appointment. It doesn't get more powerful than that. Show up late to work, moon the Prime Minister of Turkey, write crazy opinions insisting that corporations are really people—it doesn't matter! Sure, you could technically get impeached, but it's not like that really ever happens.
Once you're on the Supreme Court, you stay there. In fact, this power is so intoxicating that most justices remain on the bench long past the standard retirement age, with many literally dying before they get around to retiring. They're more power-hungry than money-hungry.
Not only are Supreme Court seats for life, but they're also the keys to the judicial framework of the entire nation. Many important votes are five to four, so any single justice's take on major issues such as abortion or affirmative action could very well end up as law. Of course, if a justice gets too far afield, that power diminishes, as five justices must agree to form a majority. However, as far as judicial power in the modern U.S. goes, this gig is as good as it gets.