I live in a state where sports betting was recently legalized and suddenly a lot of people are addicted to gambling who weren't before. There're also many people who got addicted to legally prescribed pain medication, it seems unlikely all of these addicts would have sought out heroin from a sketchy drug dealer had they not been introduced to the pain medicine first.
You'd be surprised how many heroin addicts have chronic pain problems.
Chronic pain is a strong predictor of opioid addiction, but the opioid crisis is mainly caused by wild overprescription of and over-reliance on these drugs in the medical industry. This is also why it's so specific to the US. This is a question of regulation, not legality.
Gambling addiction is not something I'm willing to comment on, because I've not studied it nearly as much as I have drug addiction. But I can see the same points applying there; regulation is also an important topic, not just legal status.
My country has a state monopoly on gambling. It's not perfect, but it's not terrible either, and I could easily see a completely hands off approach being much worse.
