In a study, you are usually interested in the relation of two things — here, ivermectin intake and COVID morbidity. The two variables do correlate in countries where parasites are a commonly infecting people. What can we claim from such a study? That the two correlate, that’s it. To conclude causation, you would have to make a double-blind study with control groups, where both have COVID (and neither have parasites) and one are given sugar pill, the other ivermectin. If that showed a significant difference between the two groups we could say that ivermectin causes decreased morbidity. But no such evidence has been shown.
What the actual studies show is simply a correlation, and we can do some educated guesses based on prior knowledge: ivermectin is an effective dewormer, the human body can produce anti-bodies against COVID, and that the immunesystem is better fighting a single thing, than multiple ones. Putting these together, we get a reasonable hypothesis showing a third variable that explains the measured correlation, through a causative mechanism (parasite hindering healing and parasite getting killed).
But surely you'll have to admit that if we do a double-blind study where all the participants have parasites we would probably get a significant difference between the two groups? Then we could also say that ivermectin caused decreased morbidity. We'd probably even say "it caused a decreased morbidity in populations with parasites, as expected". You can't just pretend people without parasites don't exist, they do. There are countries where the base rate is really high. That is where the results that show ivermectin as causing better COVID outcomes are coming from.
You've misunderstood the correlation-causation complaint. We have a pretty clear theory of causation here and the results back it up. Just because causality depends on specific conditions doesn't stop it being causal. Any medical treatment that isn't 100% effective (ie, most of them) depend on specific conditions being present - otherwise they'd be perfectly effective. Of course since the chain of causality is quite clear on this one we can conclude from the base rates of parasite infections there isn't much point taking ivermectin for COVID in the west.
Okay, why don’t we also waste money on bullshit like proving that tylenol increases intelligence (if you measure IQ tests on people with headaches)? It is just as useless. It was known that dewormers.. deworm.
There are zero new info in studies like that (unless you believe every logical conclusion requires a new study? If A is proved, should we separately prove A or B or what?), and they can’t even shut up all the idiots that still go on about ivermectin, so not even that goal is achieved..
I imagine the study-ers started with something like "hmm, these people have parasites and are getting COVID, we should see how big an effect giving them antiparasite medications has" and then went from there. Or maybe "we're throwing everything at the wall with COVID, this thing was involved in Nobel Prizes, lets try it to". Something like that. Maybe other things I can't think of. Proving up effect sizes is valuable in itself.
What the actual studies show is simply a correlation, and we can do some educated guesses based on prior knowledge: ivermectin is an effective dewormer, the human body can produce anti-bodies against COVID, and that the immunesystem is better fighting a single thing, than multiple ones. Putting these together, we get a reasonable hypothesis showing a third variable that explains the measured correlation, through a causative mechanism (parasite hindering healing and parasite getting killed).