> If we already know what a baseline untreated group looks like

There isn't really a single baseline untreated group. For a comparison between groups to be valid, all groups must be obtained by unbiased random sampling of the same population. In a clinical trial, that population is the patients served by the participating clinical center. Patient characteristics differ by time and place.

You can try to retrospectively construct a control group using a case control study design, but then you're getting to pick what control group to use, so the results are less reliable (more opportunity for human bias).

Unless a treatment is both miraculous in effect and works for everyone, it's hard to figure out if it works.