Actually testosterone supplementation for just having low testosterone is not allowed. You have to demonstrate a biological cause for the low testosterone to get a TUE. So it wouldn’t be the case someone could just temporary crash it through training and get a TUE.
So two people could have the same natural testosterone levels. One can do an MRI and find a small tumor on their pituitary which may or may not be the cause of the low testosterone and get a TUE. Other person can’t get a TUE because they don’t have a biological cause.
It does show the complexity and unfairness. Others have pointed out above drugs that are vital to daily health and how unfair it would be to ban them. Yet low testosterone is just as damaging to health and supplementation can be positively life changing, but is not always allowed.
Even then you have the issues of how much testosterone supplementation is allowed with a TUE. Should they be able to just bring their testosterone up to a median level for men their age? Or median of the top athletes in their sport? Perhaps it would be “fair” to allow boost up to whatever the highest naturally occuring number they can find is as without that they are always on an unequal playing field with those people naturally producing a lot?
It would a lot easier to just ban TUEs for testosterone supplementation. Does it suck for some people - of course, their chances to compete at an elite level are destroyed. However as I’ve said and pointed out above naturally most of us are biologically flawed in ways that will make being an elite athlete impossible, yet we don’t try to correct all of them.