A large majority of covid cases are resolved pretty quickly, so it’s unsurprising that a lot of people who take ivermectin recover well.
And there’s probably not a real problem with prescribing it for this use (aside from the fact that anecdotally it’s making people poop themselves).
But most drugs have side effects of some sort, which is why you don’t take, say, Advil for diarrhea. Sure, it probably won’t hurt (unless you’re one of the rare ones who get stomach bleeding from it) but it’s not going to help your messed up GI tract.
It’s the same with ivermectin. It attacks nematodes and insects, not viruses. There’s just not a biological reason it should work.