Do they have one for every century?

Can someone tell me why the flagship of the Federation, charged to seek out new life and new civilizations, always seems to have a 20th century Earth historian onboard?

If the crew of the starship Enterprise ever makes contact with a never-before-seen race, are the aliens ever going to be interested in Clara Bow, the 1919 World Series, Nazis, TV dinners, IBM, or the O.J. Simpson trial?

OK. Once — once — they had to fight Nazis. That doesn't count, because first contact had already been compromised. And what kind of cushy gig is "20th century Earth historian", anyway? Is it a do-nothing job like a university provost or a senior vice president?