If you arrest them you dramatically change the risk reward ratio. The fact is no one can deal drugs profitably unless they have political corruption and people look the other way.
Dealing drugs is very risky. You are not dealing with solid citizens. Many drug dealers end up getting ripped off or worse. Some end up dead. Throw in a high probability of arrest and a long prison sentence and it is not worth the risk.
You should also arrest drug users. They are vulnerable and will identify other dealers and users.
“Throw in a high probability of arrest and a long prison sentence and it is not worth the risk.”
The higher the risk, the higher the price they can demand for their product, until it is enough to justify taking the risk. You will never eliminate such a profitable industry as long as it can stay profitable.