Why shouldn't it include a bunch of treatments if they've been shown to be beneficial? How would it be better if people had to go to some other book and look up those same conditions using the names listed in the DSM in order to find out which treatments might be useful?

Doctors (at least the good ones) aren't usually going in blind and just doing whatever the DSM tell them to as if they were following a flowchart or checklist. The DSM (which I'm not even fully defending here, I personally it feel has all kinds of problems) is just a guide. It's not the only tool in a doctor's arsenal and they aren't obligated to follow it.