When their incomes don't depend on touting the myth.
It's a neural pruning mutation, with a number of variants, and its timing in a baby's development just happens to unfortunately coincide with a scheduled round of vaccinations.
Edit in: good find. Hard to cast aspersions on the Proceedings of the National Academy of Sciences
Edit edit in in: for those who don't know, thimerosal is merthiolate. The orange stingy stuff that Mom or Grandma used to paint on your cuts to keep them from infecting (vs. mercurochrome, which was also orange, but did not sting). The amount present in any vaccine, as an antimicrobial/antifungal, is minuscule compared to what would have been painted on a single cut.
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"I believe you find life such a problem because you think there are the good people and the bad people," said the man. "You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides."
-- Terry Pratchett, Guards! Guards!