It's not like doctors actually always understand what is going on; for example, I think even the precise mechanism of anesthetics isn't well understood yet.
So people volunteer to test a human approved drug for a different application. I don't believe in Ivermectin specifically, but there's nothing fundamentally wrong with that. It's what medicine does, and most researchers at universities don't have a clue either (as we now see in the entire Covid19 comedy).