Someone told them NO. You cant do that.

That made them feel mad and feel stupid.

Then someone told them that they were right and that someone else was actually the stupid one.

Then they felt better.

Better than better, they now have the secret knowledge and everyone else are the stupid ones.

There is no logic, only horse medicine.

Honestly, my suspicion is that you can easily get the stuff. I've gotten it for my dog once or twice (it's a lot cheaper than dog dewormer, despite basically being the same thing), and all you have to do is go into a Tractor supply or the like and it's right on the shelf, no restrictions or anything. This is in contrast to actual medicine, where I have to go to a CVS or the like to pay an overpriced amount for something that is more regulated, and might even be a bit of a placebo effect in the first place (look at flu meds for an example of that).

All of this mostly seems to speak more to the degrading of the American healthcare system due to costs ballooning beyond what some of the poorer in society can afford, so they are more willing to try folk remedies that may or may not work. And in the case of your grandmother, is she gonna risk paying a lot of her life savings to cure her tumors if they aren't benign (without a guarantee that it will work), or is she just gonna hope that the $10 treatment might make her feel better? If they're benign, then the $10 treatment "worked", and if they're not, then she'll get expensive treatment anyway, just probably be in a worse position to get it.

It's just one of the tenets of belief from the orange man conspiracy cult. They joined that cult and chose their special secret conspiracy "facts" to believe in.

Morons

I mean it cures worms in horses, so it must be omnipotent

It's honestly a pretty astounding medication that's been proven to prevent rna viruses from replicating in cells, and has some pretty cool anti-inflammatory properties. The main problem is that it isn't very water soluble and thus has horrible bioavailability. Basically, it would be as miraculous as people claim it to be if we can find a way to effectively deliver it to the treatment site.

Right now if you take it it primarily just runs through your digestive tract because it can't be absorbed into your blood stream. Which is why it's currently only useful as a dewormer medication.

I did some limited research on this and there are a few studies that suggest ivermectin could disrupt tumors from forming. But certainly not enough evidence to suggest using it for that purpose given the risk of adverse effects.