I've been thinking about how awful the last two US presidents have been at speaking interactively in public. Given that it constitutes a not insignificant portion of what they do (and what they've done with some regularity prior to ascending to the highest political office in the country), I'd have trouble believing it if I hadn't witnessed it for the entirety of my adult life.
Obama has a good presentation when it comes to reading what the teleprompter tells him to say (Bush could hardly even manage that), but when it comes to giving interviews or pressers, they both consistently sound like stammering buffoons:
It doesn't feel like an exaggeration to presume that a significant portion of readers could give more polished, intelligible performances than the most recent putative leaders of the free world have been able to.
It might be that politicians on the national stage are so restricted in what they say that very little can ever really sound candid or coherent. More cynically, maybe they don't actually spend any time other than when they're actually in front of the public thinking in the sorts of generic platitudes they regularly feed us and so genuinely aren't familiar with the material.
In that same vein, Jokah Macpherson adds that "the most likely explanation is that public speaking doesn't have much payoff in politics. The most successful politicians are more likely the ones that can win over the right people (lobbyists, party leaders, etc) through charm one-on-one. There's a minimum level of competence necessary but few people change votes based on good speaking delivery."
More cynically still, perhaps it's that the most successful politicians can be won over and reliably used as marionettes by said lobbyists and party leaders.