For a lot of the medical advances we had earlier in the 20th century, you didn't really need to do large-scale clinical studies to see if it was working. You gave someone an antibiotic, and they suddenly got much better. You gave people a polio vaccine, and they didn't get polio. You took someone's appendix out, and they didn't die.

Not the best examples, although you're right about appendectomies! I nonetheless agree with the broader point that decades ago there was less need for fine-grained, systematic medical studies (you were just unlucky in your choice of examples).