very are rarely those proposing taxing externalities doing so to cover the costs of mitigating those externalities, instead the money goes in to the government's general fund, or some other unrelated pet project. Also rarely the tax enough to the level that it would actually curb the desired behavior enough to effect the harm caused by the externality
So all you end up doing is making is harder for poor people to put food on the table while doing nothing to curb the externality
