I would imagine it has a lot to do with how wealthy the family was. It's easier for a single generation to waste 50 million than it will be for them to waste 50 billion.
Family sizes are also much smaller so the split of the inheritance will result in larger starting wealth for each generation.
One could blow through $50 million in a month and probably not even draw much attention. Wasting $50 billion is going to take a lot of dedication and hard work to achieve, especially if any of those assets are generating income.
Family sizes are also much smaller so the split of the inheritance will result in larger starting wealth for each generation.