I mean, you generally don't want to tie up a lot of money, each year by year, meaning that you would have a lot of frozen capital. And capitalism (which also has some flaws, but right now we are using this system) depends on the flow of money/capital. Also managing these funds would make a lot of work / administration, because someone would have to manage what goes in and out and also in what form the funds to store in. And at the point of storing money from younger people, that is not being spent, whilst using money from older people, why not just have less money stored and use the money from the younger generation for the older ones. And you go full circle to the idea that we wanted to solve. Each system has its benefits and flaws, some of which are greater, which outweigh other, smaller ones. Sometimes the solution can be something completely different.