That's because it's just not true in any meaningful sense. What happens is that the wealth is often transformed and split between separating couples, but it mostly stays in the same class.
The idea that generational wealth is lost is a great story though. It's part of the "American Dream". Anyone can do it. The "self-made" man. Rags to riches. If you spend any real time with the wealthy people—almost anywhere in the US—you know this isn't true. It happens, sometimes, sure. What you DO see is plenty of "millionaires" who saved 50% of their income for 45 years and get to retire at 65 with >2 million in the bank. There are tons of these people. This is what the wealth planning industry creates and it's what skews the statistics about these sorts of things. Poor for a lifetime for a taste of wealth at the end.
But man, the idea that, "70% of generational wealth is gone by the 2nd generation" is literally a joke. I can imagine it as the punchline to a joke where management is trying to convince to labor to work harder.
It's so easy to be wealthy when you grow up with money. And it's easy to stay there as long as you don't fall into a serious addiction of some sort. And even when you lose a bit it's easy to get back because the connections you have basically guarantee you income. Being born into wealth is damn near a safety net that lasts a lifetime. If you can earn the respect of a just 5 or 6 people in wealthy families you never really have to worry about money. And truth is, once you know 2 or 3 you end up with a dozen contacts. Your next venture is a phone call away. There is always a job for "talented people". That's the reality that people really don't talk about because why would they. It just pisses anyone off who isn't at that level.
When I see people talk about how most generational wealth is lost by the second generation what I have to assume they are talking about is someone who inherited $2 million or so when their parents died. Not because their parents were wealthy, but because they spent their entire life relatively poor (despite having some assets), saving for a day that never came, and then died at 70. So their kids grew up relatively poor, and suddenly at 40 they have a few million that they quickly piss away. That's a scenario that happens all the time that people are familiar with. That's technically generational wealth. But when I think about generational wealth I think about families that own land, businesses, assets. Families that own city blocks.
Truth is, if you show up at the elite country club for the first time at 65 no one is going to consider you wealthy, especially if you don't have the stories. The real wealthy people have been there since they were 15. Wealthy people have a lifetime of wealthy people stories that they like to tell each other. Real generational wealth is its own society.