The issue with diagnosis and why it is done so "quickly" is because of insurance companies. Insurance companies REQUIRE a diagnosis to pay for services. That means that the client who comes in for an intake session, the therapist has to have something to put down on the paperwork after the first session. Therapists know that the actual diagnosis might end up changing as they find out more information. They have to have at least one disorder that is the 'primary' diagnosis. After that there are secondary diagnosis that is anything that is not seen as the most influencing disorder. IE if someone has major depressive disorder that is significantly impacting their life and social phobia that keeps them from being able to give presentations in class, the depression would be considered the primary diagnosis and the social anxiety the secondary diagnosis.

There are also diagnosis that can be listed as 'Rule Outs'. This means that the therapist thinks that the diagnosis may fit but does not have enough information one way or the other. This may be because there was not enough time to fully investigate the diagnosis given time constraints, or because the client tells the therapist that they have had a prior diagnosis of X and the therapist is not able to confirm that at the time.

The thing with diagnosis is that there are a lot of symptoms that overlap between diagnosis and diagnosis that are often found together (ie anxiety and depression) so this makes it harder to diagnose. So some times it takes more than one or two sessions (or a lot more) to figure out an accurate diagnosis. The other issue for example is with bipolar disorder. For most people (not all) a depressive episode happens before a manic or hypomanic episode. Given this, there may be a first diagnosis of depression that then changes to bipolar disorder after there is evidence of a manic episode.

So there are a lot of complications and details when it comes to diagnosing people.