Defamation of public figures is governed by the "actual malice" standard: the person making the statement must either have known that it was false at the time they said it, or must have been acting with reckless disregard for the truth (meaning they had serious doubts that the statement was true at the time they said it). The First Amendment bars a public figure from winning a libel suit unless they demonstrate that the defendant fell in one of those categories, because any lesser requirement would discourage people from speaking on topics of public concern for fear that they might say something wrong and be sued for it. The standard was first applied for public officials in New York Times v. Sullivan, and later cases have extended it to public figures in general.
If someone genuinely thinks Obama was born in Kenya, it is not libel for them to say that. Even though you could argue that any reasonable person should know that's wrong, it's not enough -- the defendant had to have known it was wrong or seriously doubted it. Even if your sole basis for claiming he was a gay prostitute is that you heard a rumor from a friend, if you actually believed them, you can say he was. You aren't required to check Clinton's book to verify a quote before repeating it; if you read it on a website and had no reason to think they were lying, you can say that the quote was in there.
It is extremely difficult for a public figure in the United States to win a defamation lawsuit. This is the system working as designed; a public figure who wants to correct lies being told about them can put out the correct information (which is easier for them than for most people), which is preferable to government action (and libel judgments are government action, because they involve a government officer ordering you to pay someone else money and/or do and/or not do something).
