Testing drugs on insects is much easier, faster, and efficient than testing in humans.
This answer has been posited by a few other posters. However, given fixed resources for the FDA and other regulatory agencies, there are only so many applications that can be reviewed per unit of time. Thus, what is needed to obtain more approved drugs is not more clinical trials or easier clinical trials, but rather a higher proportion of meritorious applications.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”