Because that’snot what the graph claims, and it is definitely not what the graph implies.

The graph says that there has been a 400% increase in the prevalence of autism. That’s not true, and is unsupported by the evidence. There has been a marked increase in the effective diagnosis and therapeutic interventions, but autism was largely undiagnosed and under-reported for almost all of human history. We’re still improving and refining the diagnostic criteria, and any changes in the number of cases should not be suggested to support any causal relationship with anything.