Frankfurt calls bullshit "a greater enemy of the truth than lies are." Why?

Because the liar still respects truth enough to track and oppose it, while the bullshitter abandons the very distinction between true and false — and doing it habitually erodes the ability to care about facts at all.

A liar, paradoxically, is responding to the truth: to insert a falsehood at exactly the right spot, they must know what's true and work around it. They accept that there is a fact of the matter. The bullshitter rejects that whole frame: they "pay no attention to it at all."

The deeper danger is corrosive. Frankfurt argues that excessive bullshitting unfits a person for telling the truth: making assertion after assertion with no regard for accuracy wears away the very habit of attending to how things really are. Lies leave the boundary between true and false intact; a culture awash in bullshit dissolves it — and "the conduct of civilized life... depend[s] very fundamentally on respect for the distinction between the true and the false."

Tip: Brandolini's Law captures the practical cost: the energy needed to refute bullshit is an order of magnitude greater than the energy to produce it.