Didn't stop ol' Jeb from trying, and hell, there's always the example of Nixon. As for the Clinton vs Obama camps, do you really think she was happy with the SoS nod after Obama found the weakness in the caucus system and used it to take what would, definitely, have been Her Election? Obama was the only reason that was even a contest; Hillary would have slaughtered McCain, not just beat him. He could have been the 2016 nominee, and in all likelihood would have been a better and more effective president if he'd spent another eight years learning how Washington works (although eight years in the governorship would have been even better, except for Illinois governors' unpleasant tendency to end up in federal prison).
Look, maybe you know some people on the DNC, and I sure don't. But that's how it looks out here in flyover country.