Let see . . . what are the differences between testosterone, anabolic steroids, and amphetamines? Hmmm . . . I dunno . . . maybe the body does NOT need anabolic steroids or amphetamines and that both are non-natural, not produced in the body substances.
Now, if you're lacking in say insulin like many diabetics do you take insulin or do you make the same argument that you made? Some people need replacement. The same way that some could use testerone replacement. I'm not sure why there is so much opposition to this concept.
Sure, insulin levels when too low cause certain measureable and identifiable changes and replacement corrects or controls them. Lots of diabetics, though, are not too low in insulin, but do not use it properly at the cellular level and some actually have elevated levels.
With respect to testosterone, some men are clearly abnormally low and benfit from testosterone. However, what is abnormal is not clear as there is a large range of normal levels. Use of testosterone has some dangers. It is not a benign thing to use.