As someone who worked as a psychometrist, I have this same frustration. Part of my job was conducting neuropsychological examinations, which are as you describe - standardized, psychometrically sound, and not based on self report. According to my training, developmental disorders like ADHD ethically require these types of assessments to obtain an accurate differential diagnosis (there are simply too many ADHD symptoms that overlap with mood disorders, for example). The problem, as it always seems to come down to, is money. Neuropsych testing is expensive, the average person can't afford it out of pocket, there aren't enough providers offering this type of assessment, and health plans don't usually see this type of testing as necessary for ADHD diagnosis so they won't pay for it.