It's not, though. At all. It literally contains morphine. It's harder to judge the dose. You will develop dependence every bit as quickly, and you can die from respiratory depression if the dose is too high.
Unlike purified morphine, raw opium can be selectively bred--or engineered--for qualities desired by humans. One such quality might be lower incidence of accidental overdose.
So perhaps the poppy also produces a substance that creates an aversion affect (probably due to extreme nausea) when it reaches a certain concentration in the body. If you approach an overdose, you might start puking your guts out, and wouldn't even be able to look at a 1928 O'Keeffe painting without clamping your jaw shut.
With purified morphine or heroin or methadone, you might just fall asleep and never wake up.
But the raw plant material might also contain something you don't know about that erodes the septum in your heart, and you die from chronic use instead of overdose. That's just one of the risks with whole-plant drugs.
Generally speaking, we have attempted to eliminate hidden side effects by using purified molecules, and attempted to overcome overdose by using prescriptions from trained experts. That isn't necessarily the best or only way to run civilization's pharmacy.
What would create that aversion effect? Thebaine? Anything's possible, I guess – but I haven't seen much evidence that there's any component of raw opium that can substantially slow or stop the respiratory depression and/or reinforcing effects.
I'm not saying it's not addictive, or that it's not dangerous - it is.
Morphine is however just one of the many alkaloids that make up opium - together they have a different effect than morphine alone.
I can't quickly find a reference beyond Wikipedia[1] that mentions opium being less addictive than, for example, morphine, but I read a lot of papers on drugs and have read this before. I'll try and find some and post back if I do.
A bunch of years back, there was a minor internet epidemic of people ordering poppy pod arrangements off of eBay and brewing them in to tea. It proved every bit as addictive as abusing morphine. There are a lot of plants out there that have a safety-favourable agonist/antagonist mix, but I'm not sure opium is one of them.
