What is their vested interest in a generic, cheap, decades old, off patent drug? Vested interests tend to be in new, patented treatments. That's where big pharma profit is. So let's try a rephrase: Many people pushing patented treatments have a vested interest in proving off patent alternatives are unnecessary.
For example, here's Merck warning against using Ivermectin for Covid-19:
https://www.merck.com/news/merck-statement-on-ivermectin-use...
Then a few months later, "Merck Announces Supply Agreement with U.S. Government for Molnupiravir, an Investigational Oral Antiviral Candidate for Treatment of Mild to Moderate COVID-19"
https://www.merck.com/news/merck-announces-supply-agreement-...