Probably because of social pressure. In Britain and the USA, older girls like pink more than boys do, but they could already have been influenced by expectation. So studies have tested one – to two – years-olds by using the ‘preferential looking task’, which measures what the children like to look at the most.
The studies found that preferences for toys differ by sex, with boys looking longer at cars and girls at dolls, but preferences for colour do not. So perhaps it’s not surprising to learn that back in 1918 the trade publication Earnshaw’s Infants’ Department wrote that babies’ clothes should be pink for a boy and blue for a girl.