That the parents of severe cases eventually pass away and unless they figure out to take the kid with them, he is condemned at best to a life in mental health institutions - and usually they make One Flew Upon Cuckoo's Nest look like Teletubbies.
Add to that more and more people are single kids and usually born out of geriatric pregnancy (which also increases the chances of autism somewhat) - aka above 35, so they really are alone.
There are very good state and society interests in preventing autism. Mental disabilities are way worse than physical in today's society. Thankfully not every case is severe. But severe one's do exist.