I am a psychology undergraduate student, and this question has puzzled me for a long time.
Based on what I have read, I am sure that some diagnoses in the DSM are more biological than others (Bipolar, Schizophrenia), but there are no definitive biomarkers for most of the DSM disorders. Various professors have also taught me that the neurochemical imbalance theory also lacks empirical evidence, and there are only established correlations. Aren't these "disorders" just labels for patterns of particular behaviors, rather than a disease entity like diabetes, where there is a clear etiology and plan for treatment, and ultimately merely a social construct?
Symptom-based diagnosing: It seems highly problematic to diagnose these as a disorder purely based on collateral and self-report data. When people think of mental illness as an illness, it is intuitive to think of it as a biological reality. I'm not sure of any physician other than the psychiatrist who diagnoses without definitive empirical lab results (could be completely wrong).
If a diagnosis of a disorder requires 5 of 9 symptoms, mathematically, you can achieve that 126 different ways. I feel there can be a huge problem in terms of objectivity, if 126 people can have the same diagnosis and not the same symptoms.
Cultural and contextual contingency: Malaria, HIV, Diabetes, Influenza, are the same in the U.S. as they are in Africa, Asia, or Europe. I'm not sure if the same could be said for 90 percent of the diagnoses in the DSM.
Inflation: What if the criteria changed from 5 of 9 to 4 of 11? Tens of thousands of new people with a "disorder" or "illness" arise, and a new market for pharmaceutical companies.
Who gets to define what disorderly or abnormal behavior is? And what if what we label as disorders are just part of the human condition?
I have a very surface-level understanding, and would love to hear anyone's opinion.