"Neurotypical" is not just a word for people who are not autistic. It's a word for people who fall roughly in the (statistical) normal range for cognitive functioning and neurology. It started in the autism community, but it's a word that is useful for other groups, too.
If you want to say, "People with ADHD are not (usually) on the autism spectrum", you may say, "People with ADHD are (usually) non-autistic." "Not autistic" covers neurotypicals, as well as people with atypical brains who do not have autism; we didn't need to make a new word to cover people who weren't autistic, because we already had an easy way of saying that. But to talk about people whose brains weren't remarkably unusual, without using the term "normal" (the connotations of "normal" and "abnormal" carry too much baggage to be used as neutral terms), we needed a new word. So we use "Neurotypical."
We're not the only group that does this. For example: Cisgender (opposite of transgender); heterosexual (opposite of homosexual); able-bodied (opposite of physically disabled); sighted (opposite of blind); hearing (opposite of deaf/Deaf); people of color (opposite of white); indigenous (a group of people who already lived in a place when another group invaded or settled there). Some words are adapted from words that already exist. Some words are invented. They're often ways to talk about things without using the term "normal", because "normal" implies quite a lot of things that we don't mean when we talk about the majority as opposed to the minority.
Because the human brain is so unique and so different from person to person, even "neurotypical" is a rather broad range. Neurotypicals may become artists, surgeons, politicians, or soldiers; their brains are flexible and can train to deal with all of those things. They have widely varying personalities, from the introverted bookworm to the charismatic cult leader, the impulsive adrenaline-addicted extreme athlete and the careful, detail-oriented archeologist. So we can't say they're all the same (that would be silly, nonsensical, and more than a little offensive)--but they all think in a way that's close enough to average that they fit into the way the world works without having to make major adjustments. They are also valuable as communicators and connections for the neurodiverse, because their cognitive flexibility and social talents allow them to--if they desire to do so--communicate with people who are very different from themselves. This allows the neurodiverse to contribute to their societies, when neurotypicals accept and include them, and strengthens the whole society as a result.