It clusters symptoms "without underlying cause" because we don't know the underlying cause. If we wanted to go for a fully "physical mechanism" approach to mental health, we could just end any treatment attempts and say "come back in a few decades or perhaps centuries".
But we do know that certain symptoms tend to show up in clusters, and that patients in certain clusters tend to respond to certain drugs with an above-placebo level of effectiveness.
The same is true for a runny nose: It can be caused by any number of viruses (there is no such a thing as "the" influenza virus or "the" common cold virus), or allergies, or irritants like pepper spray, or a problem with the body's ability to regulate itself, or something else entirely – but going through the effort to test for every virus, allergen, irritant, and whatnot is wasteful, if all the patient needs at that moment is some nasal spray to breathe properly again. Incidentally, a runny nose is (or perhaps: was) not a common symptom of Covid, so unless other symptoms more indicative of it show, there may not a good reason to test for it, or prescribe medication specific to it.
If more symptoms accumulate over time, or the symptoms don't go away, you then probably can go back to your doctor, will get a different diagnosis and possibly a different prescription. The same is (or at least: should be) true for mental issues, where you might switch treatment over and over until something is found which actually helps your symptoms. Is this a flawed process? Frustrating? Absolutely. But is it "worthless"? I don't think so.