When you have three, four, five kids it's not as much a problem if one or two get seriously, long-term damaged hurt as a result of some childhood accident, somewhere on the LGBT spectrum, infertile, in physically exhausting manual labor jobs or in war as there are still other children to keep the family line alive.
With only one child, the pressure on that one child to turn out "perfect" is much, much more immense - and parents will naturally helicopter around to ensure that their one and only offspring achieves the best they can, whatever the cost.