A: They do this because they cannot get a valid assessment. Some lack access to mental health care, especially in the United States. Some are being asked to pay such high fees that they would have to choose between an assessment and their monthly food budget. Some know that they could lose their jobs or be denied child custody if they were officially diagnosed. They are doing the best they can with the information they have.
When you lack medical care, have just fallen down the stairs, and see that your arm is bent the wrong way, it’s not unethical to diagnose yourself with a broken arm and splint it as best you can. What’s unethical is that somebody has denied you the right to see a doctor, or made broken arms so stigmatized that you fear anyone knowing you’ve broken yours.
ASD is a real disability, and most of those who self-diagnose have a real disability. Or do you really think they would identify themselves as having ASD if there were no significant impairment for them to worry about, no social confusion or communication impairment or sensory processing disorder? Sure, some of them are wrong about what exactly’s going on; maybe they say it’s autism when with a bit more study they’d realize it better matches ADHD, or social communication disorder, or schizoid personality disorder. But then, some professionally diagnosed with autism are also misdiagnosed. Considering that the professionals get all the obvious cases because their parents bring them in, I think self-diagnosed people are doing pretty well when it comes to accuracy.
If you’re upset about this, then start advocating for universal access to mental health care and the removal of stigma around autism and disability in general. Until you’ve done that, don’t disparage those who self-diagnose.