Why? Should we first suspect carrots of making people grow red hair, suspect eggs of causing pregnancy, and suspect Left Twix of causing left-handedness?

We don't know the cause of autism. We do know that autism has a heritable component, with significant rates of both siblings having it (which could be explained by environmental factors) and both parent and child having it (which cannot be explained by environmental factors). Surely it would make a lot more sense to suspect a genetic component first?