A theory... It takes time to create wealth, and a lot more time elapsed before the current generations than has elapsed (or will) during the current generations, thus perhaps it’s not surprising that as time goes on, most of the accumulated wealth around us is from previous generations.

Furthermore, when a person dies, their wealth can either be destroyed/lost, inherited by heirs, bequeathed to some institution, or absorbed into the state/public... by measuring only “private wealth” as TFA does, we don’t see those latter two routes for the flow of wealth across generations, thus biasing our view toward the inherited component of wealth.