The computerized test does more than diagnose.
It measures exactly how long a person’s attention span is, to the minute, and how figitty he is. This is more accurate and informative than through observation.
The fact that the test does not reflect daily activities or that it might be boring, which it is not (it is like a video game) is irrelevant to it being considered by professionals to be an accurate method of measuring the presence of ADHD.