It's because they're hooked on opioids. How did they become hooked? They started with legally prescribed opioids. Sometimes those meds were prescribed to them. Sometimes those meds were prescribed to a family member or neighbour and handed on or stolen.
It's because the US is awash with opioids that were legally prescribed. The US massively over-prescribes opioids. That's why the US used over 99% of the world supply of hydrocodone.
Americans are prescribed 6 times more opioids that people in France or Portugal.
Here are some details: http://www.painpolicy.wisc.edu/opioid-consumption-data
And we know Americans start with meds that were legally prescribed because: https://www.samhsa.gov/data/sites/default/files/NSDUH-DetTab...
Look how many people are getting meds for free from friends and family.