Because some of the symptoms of mental retardation look the same as autism.
Fixations (i.e., spinning wheels, cartoons, animals, textures, et cetera)
Meltdowns
Need for Routine
Non-verbal
Obsession with a singular topic
Repetitive behavior (i.e., Rocking, Stimming, et cetera)
(Maybe more?)
So, to the untrained eye mental retardation and autism look the same.
(Dude, everyone is ignorant about something; so lighten up!)
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The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.