Why is insulin not an over the counter medicine? Would having it OTC cause problems on the scale of alcohol? What's the "value add" of having a doctor renew an insulin prescription?
A better response is that several older, more difficult to manage versions are available over the counter.
Weight trainers can and probably already do (ab)use insulin to maximize their gains by manipulating blood sugar levels during workouts.
self-diagnosed and self-medicated diabetics showing up at hospitals that have no indication of this and pump them full of glucose.
Are you saying that insulin has to be prescribed to prevent murder?
Second because the peculiarities of insurance often mean prescription drugs are cheaper to the patient than OTC drugs. Making insulin OTC might well make it more expensive.