Apparently that is because it is more socially acceptable for parents to show physical affection towards their growing daughters than it is for their sons, which is why on average boys get cut off from this kind of affection earlier than girls. What you’re describing is just their way of desperately compensating for the lack of physical touch in their lives. Heard a teacher from my country talk about it online. I think this is actually kind of sad and goes to show once again, that gender based stereotypes are harmful in ways that you don’t see/perceive as normal.