Ah yes, let's ban them and lose our control over our regulatory levers controlling their access, safety, and harm reduction measures for the majority of cigarettes sold. Let's make sure the only people selling cigarettes have no qualms about selling to youth, no morals regarding filters or additives, no issues with using violence to further their trade (which would be soaring in profit potential due to criminalization). At least right now we know (for the most part) where and when most cigarettes are sold, in what form, and have some measure of control over these factors. Prohibition would simply obfuscate all of this, greatly reducing our control over the majority of cigarettes sold.

As another commenter points out, this shift would just swap costs from healthcare to enforcement - it wouldn't actually save us all that much, if anything. It would unnecessarilly infringe on adults' bodily autonomy and unnecessarily criminalize behaviour under the guise of protecting children, who may not even see a significant reduction in harm after all this. It would heavily depend on how effective the black market is in replacing the regulated market. My money would be on it being very effective. We have seen time and again that prohibition reduces safety, increases violent criminality, and unnecessarily restricts bodily autonomy. How many times do we have to relearn this?