Another factor: Many doctors and hospitals are rated according to patient satisfaction, with reimbursement rates from the government partially based on those satisfaction surveys. And in many cases, if patients have any pain whatsoever, they rate the doctors and hospitals poorly, and therefore the doctors and hospitals get a lower reimbursement rate from the federal government. So doctors and hospitals are incentivized to make patients have no pain whatsoever in recovery, in order to have higher reimbursement rates, even if it means addiction in the long run.

Source: Many doctors in my family who talk about their experiences.