For the democrats, putting forth imbeciles as their candidates is a feature, not a bug. They understand and favor the idea that the US is no a technocratic oligarchy ruled by the bureaucracy. They don't want some egomaniac politician getting ideas into their head and mucking things up by taking independent action, even if they are nominally on the same side. Thus the rise of the empty-suit figurehead political class.
“They don’t want some egomaniac politician getting ideas into their head and mucking things up by taking independent action”
David Icke (much ridiculed around here by some) claims that the idiot politicians serve the purpose of convincing the masses that they need to be ruled by “wise elites” of a world government since the local politicians are clowns.