A popular theory among self-proclaimed paychonauts, or people that like to experiment with drugs, but mostly psychedelics, is that using mind-altering substances could lead to ideas or insights that could be (considered) a threat to the status-quo. This is why the powers that be often demonize all drugs equally.
As others have pointed out, many people don't really like to think about the systemic issues in society that drive people into poverty, homelessness or other problems that often go hand on hand with drug addiction.
It should be pointed out, though, that some drugs can and most likely will ruin your life. The first rule of experimenting with drugs is never, ever try heroin (or other opiates).