> if the matter really were so clear, then why, over the last ~200 years, did every country introduce a prohibition-based stance, and why has no country switched to tax-and-regulate?
First off, you do have countries switching over to tax-and-regulate, Canada, for example, some U.S. states, countries in Europe (Germany, Netherlands).
But to answer the question of countries going to prohibition-based instead of tax-and-regulate - off the top of my head
1) US-led world order + US deciding to ban a bunch of drugs for political reasons (I hear, from multiple sources that discriminating against Mexican immigrants was one reason cannabis was outlawed, Reagan and his whole 'War on Drugs' schtick...)
2) tradition - have you noticed that, also for the last ~200 years, basically all world leaders, from French presidents to the Emperor of Japan, will dress in the style of the English monarch George IV (and, more precisely, in the the style George IV copied from Beau Brummell)?
We ban drugs because that's what we're used to doing. Our leaders dress in suits because that's what we expect them to do.
3) naive ignorance - much as how, for hundreds if not thousands of years, "sacrifice a bull" was considered the state of the art in ensuring that there would be water for the crops, people growing up with no additional knowledge will often arrive at "ban it" as a logical way to handle a social ill, even if, as those who have suffered the negative consequences [0][1] know, "ban it" generally just makes the problem worse.
[0] https://en.wikipedia.org/wiki/Nuevo_Laredo
[1] https://en.wikipedia.org/wiki/Mexican_drug_war
Source [1] notes
"Analysts estimate wholesale earnings from illicit drug sales range from $13.6 to $49.4 billion annually".
Not the illicit part. The market is there, even if outlawed. Dozens of billion of dollars per year, and if you tax-and-regulate, all of a sudden you've got billions in tax revenue to help you "respond to social harms, such as the condition of opium addicts".