There is a lot of research showing that ivermectin has broad effectiveness against a number of viruses long before the pandemic and including SARS and MERS. There is also strong research (published in Nature) that shows ivermectin to be highly effective at blocking some aspects of a SARS Cov2 infection. The main concern is getting a high enough concentration in the body for it to be effective.

There are additional antiparasitics that also appear quite effective against COVID, some of which are being reformulated and brought to market.

There are also a host of treatments that were demonstrated to be quite effective against COVID via placebo controlled double blind trials that were ignored. If something is off patent, it will almost never be repurposed for another indication by a US pharmaceutical company.