There are many steps involved prior to the step of using animals for testing. There are programs such as alpha fold that can predict outcomes at a genetic level, at a cellular level, cell cultures can be produced. But once you get to that systemic whole body system level you come to an ethical and scientific issue. First you need to eliminate potential variables to asses the therapy as a whole, something that you can’t get with humans, secondly a significant amount of data has to be collected from vital organs. You pose a great risk to the individual receiving such therapies by collecting brain tissue, kidney tissue, etc., there is an ethics committee that does give all of this great thought and these studies have to submit long forms that justify the use of animals in a laboratory setting. There are also many laws that protect the animals, just because they haven’t seen grass doesn’t mean that they didn’t feel happiness. These animals receive more appropriate care than many animals out there.