@SuperSocks because psychiatrist and psychologists look at the whole person, not just the traits. They identify the traits, the impact the traits have on you and your life, your social skills etc. They apply that to the criteria required to take you across the threshold from NT to autistic, combine that with professional judgement plus discussion with another colleague - I've only been through my children being diagnosed but in both their cases, it was more than one mental health professional each time to reach a conclusion.
Obviously this is all the 'I've no idea but I assume it's along those lines' viewpoint of an unqualified health professional.
It's a bit like saying, everyone who feels sick every day, everyone who has put on a bit of weight - must be a bit pregnant. It's literally as ridiculous as that. A few symptoms do not make a diagnosis. A few traits do not make you autistic. It's insulting to autistic people to suggest everyone is 'on the spectrum' because that implies that autistic people are actually neurotypical. Which they aren't! It also implies all neurotypical people are autistic, which they aren't. It's really a ludicrous statement if you think about it.
Unfortunately autism impacts people in such different ways and to different extents, it has to be described as a spectrum. From that people have jumped to the erroneous conclusion that the spectrum goes in a straight line from 'completely normal/neurotypical to autistic' or from mildly autistic, to extremely autistic. It doesn't, it goes from autistic - with these particular dominant traits to autistic with different dominant autistic traits. And it's not really a line. A circle better describes it (I've attached a couple of pictures that I think explain it quite well) .
I hope that long waffly explanation helps!