Even if it's already approved for human use, we still need to test for efficacy before prescribing it to everyone. See also: remdesivir, favipiravir, Prezcobix, chloroquine and ruxolitinib, among many others.
There even have been several blinded trials, that were discontinued mid-way, because the drugs being tested were so effective.
To continue giving placebo and denying obvious cure would be unethical, and so the trial was stopped, placebo arm cancelled - and everyone got the actual medicine.
in the “just try it and see scenario”, you offer it as a possibility, and everyone who ops in, gets it.
it might not be as scientifically rigorous, but that's a big difference to the patient who suffers from receiving a placebo instead of the real deal