This is because only children have a lifestyle that people from large families subconsciously wish THEY HAD, Those from large families have little or no resources They have little monies for the rudiments, if that. They also have little or no parental resources. Parental involvement in their lives are nil or close to nil. There is no such thing as individualized, quality parental attention, time, & care in large families. Children in such families learn to raise themselves or each other. In fact, the primary relationship in large families is never parent to child but sibling to sibling.
On the other hand, only children have the luxury of individualized, qualitative parental love, time, care, & attention. They don't have to vie w/siblings for parental resources. There is also more disposable income in one-child families so that the child is exposed to travel, dancing classes, fine restaurants, & other intellectual/cultural activities which are sorely lacking in the large family environment. Children in large families know that they are living in scarcity & lack in all areas. When they see only children having the opportunities that they don't have, their reaction is anger but instead of turning that anger to their parents, they displace their anger regarding their deficient lifestyle towards the only child.