Why is there no widely accepted chemical antidote for paraquat ingestion?

No agent can reliably reverse the established cellular damage

The medical community lacks a specific antidote for paraquat because, once the toxin has entered the bloodstream and initiated the redox cycling process within tissues, there is no pharmacological agent capable of reliably stopping or reversing the resulting cellular destruction. Unlike other poisonings where a specific antagonist, such as naloxone for opioids, can instantly block the effects of a toxin, paraquat damage is a rapid and relentless cycle of oxidative stress that destroys cellular components. Because this cycle is self-sustaining and causes immediate structural damage to vital organs like the lungs and kidneys, treatment must shift entirely to supportive care rather than attempting to chemically neutralize the circulating toxin.