Well, as an autistic diagnosed person, NO, the diagnosis does not tell us "Why" anything is happening. It is a description, not an explanation. In fact there is much discourse on all levels about "trying to find the cause of autism." However! What this discourse overlooks is: if a "cause" were ever "found," that would be a HUGE redefinition of what autism is!
Think about it: if someone has been understanding themselves as autistic their whole life, been diagnosed, found Autistic community, been involved in self-advocacy for acceptance and accomodations - AND THEN, someone says, "We Have Now Found The CAUSE OF AUTISM." But the person in question, turns out to not have whatever that "cause" is! Does that mean they are "no longer Autistic?" No - Their lifetime of experience is still their lifetime of experience!
So, in conclusion - autism is an umbrella term, fairly subjective, that describes a cluster of experiences stemming from LOTS OF DIFFERENT sources & causes - take it or leave it based on whether it seems a useful descriptor to you or not. You know yourself better than any doctor ever will.