Selling cocaine/heroin at the corner grocery shop would bring in billions of dollars of sales. Just imagine tens of millions of people buying their daily does.

So if following the money is the answer, why doesn't this happen?

Because it's really deadly and addictive. Legalising it would bring untold number of deaths. We've been there before, opium, laudanum and heroin were available without prescription one time...

The grocery stores already sell highly addictive and thus highly profitable substances.

IMO, it's not follow the money, it's follow the power. The power to lock up anybody doing a common thing because you don't like them for some reason is intoxicating. Maybe power is the most dangerous drug of them all.

Because not selling them at the corner grocery shop brings in hundreds of billions in sales.