Ending child marriages isn't only good for the young girls forced into such arrangements but also for society as a whole. Girls are more likely to have complicated pregnancies, get divorced, and remain impoverished if married before 18, leading to a repetition of the cycle for future generations. If governments want to produce thriving economies, helping girls complete their secondary education before starting a family will go a long way in achieving that goal.
The idea of "human rights" was born out of the UN's quickly-formed Declaration of Human Rights following World War Two. While many people around the world may agree on certain rights, they don't actually exist naturally but rather come from negotiated agreements between governments. Though cultural aspects like forced child marriages are rightfully abhorrent in the minds of both Western and non-Western peoples, that doesn't mean Western institutions should establish a hegemonic set of rules for the world to follow.