Because you’ll lose when your neighboring country doesn’t apply that tax, and then you’re forced to impose a tariff on their products so it doesn’t ruin your ec

Because you’ll lose when your neighboring country doesn’t apply that tax, and then you’re forced to impose a tariff on their products so it doesn’t ruin your economy, but then you realize that you’re effectively taxing your citizens with that tariff while you’re incurring all the costs of your neighbours pollution.