I have my doubts about IVs in practice, but I agree they theoretically are an acceptable substitute. The problem is we conduct science on systems we don't full

I have my doubts about IVs in practice, but I agree they theoretically are an acceptable substitute. The problem is we conduct science on systems we don't fully understand. Because we don't fully understand it, it's easy to miss important information we didn't know is important. The double blind with random selection allows us to control for all important information, determine causation, and is the best known method for controlling for bias. We don't have to worry about missing important variables because the nature of random selection means those variables are randomly distributed.