Why are autism and ADHD “mental illness” when they’re actually more closer to things like Learning disabilities than a true mental illness? ?

No, they are not considered as mental illness.. At least not by the professionals... 

I have never heard autism described as a mental illness, do you have a source?

out in the real world,there are as many definitions and descriptions  and treatment for autism as there are  chicken and rice recipes I E medicine is seldom science,seldom is there agreement among psychs psychiatrists and  doctors what it is or how to treat it let alone identifying it teachers or parents may notice slowness in learning lagging motor skills,speech,then spend years and a lot of money to help the kid

There are classification manuals that list things under different categories, based on different criteria; and most doctors I believe use two major manuals as a way to classify your autism (like the DSM-V).  It depends on which one they use, and which types of autism you have; as well as the human factor (what kind of doctor you have), as to what your diagnosis it ends up being.  This means that it is subjected to inconclusiveness (meaning there is always a small chance you have the wrong diagnosis)!

The way that all 'brain function illnesses' are classified, is also something that cannot be strictly defined; and such classifications are controversial in nature.  While you may believe it is wrong (which is justified), someone else may not (which is also justified); and so we can only hope that the doctors and manuals are correct.

These classification systems update and change all of the time; and I myself have autism and have had my condition re-classified and my diagnosis was also changed.

This is not that unusual in the field of psychology and psychiatry; and is a very common problem.  Please don't take it so hard.

Autism is listed with the behavioral health disorders and conditions because it is a neurodevelopmental disorder. ADHD is a learning disability and a disorder of impulse control. It is not considered to be a mental illness in the traditional sense of the word. Down Syndrome, named for British physician and researcher John Langden Down, is a genetic disorder that can and often does cause intellectual and physical disabilities.

Dyslexia is a learning disability as well. As for dyspraxia, dyscalculia, etc., those are considered to be learning disabilities, not mental illnesses- at least not in the same sense that something like Schizophrenia would be. They are classified WITH other mental illnesses, however, because they involve thought processes and impact behaviors.

Mental illnesses are NOT always brain disorders, STF. Yes, they can have a component that directly impacts the brain, but they are not always organic (meaning that they involve the brain) in origin.

To add... STF, ADHD is very definitely a learning disability. It's classified that way in all of the coding manuals, but more to the point, it's considered to be an LD under the disability laws that govern most states.  And calling it so is NOT the same thing as saying that a PHYSICAL disability like blindness is also an LD. Blindness has multiple causes, and it's very possible for people to be blind and yet not have any problems learning or with their intellect. Just look at someone like musician Stevie Wonder if you want proof of that. He's been blind since childhood, but he has totally normal intellectual functioning.

I have never heard autism described as a mental illness, do you have a source?

I've never heard of autistic spectrum disorders referred to as mental health issues, they are brain disorders.