I am going to hate myself for diving in to this thread, but I do believe you are generally sincere in your questions.
I think the vast majority of clinicians (physicians and veterinary clinicians) want every available diagnostic and treatment modality available to be on the table. That said, we live in a very litigious society, and medical malpractice is many lawyers' best friend. A patient dies while use of clinical proven and relevant modalities is a whole lot less easy to sue than a patient dying with unproven or non-FDA approved therapies being used. Though "defensive medicine" is not as widely exercised as many think, the thought sits in the back of most if not all clinicians' minds when making those assessments. It is easy for a lay person, such as yourself, to say "lets try anything," than it is a trained clinician, who has the responsibility to maintain rational medical judgement and recommendations. Clinicians are not infallible, and there are dirtbags out there (as there are in any profession), but demanding unproven therapies be used is just not going to move the needle.
I have heard more than a few anecdotal situations where ivermectin was used and may have been the main source or even just a significant cause for improvement. Like 'tologist said, they got lucky, and (seriously) good for them!