I think their rationale is they know Biden's completely cooked, Kamala's almost as unlikable, and nobody on their shallow bench wants to become the candidate knowing what a deep hole they're already in. Anyone who has the resume and positioning to be a potential replacement candidate that might actually have a chance against Trump is inherently going to be a careerist ghoul that doesn't want the blame of being the new captain at the helm of a sinking ship to land on them and ruin their chances of being the nominee in 2028.