Because the government is the source of money, it doesn’t make sense for the government to save people’s money.

Consider a hypothetical island where everyone eats fruit and people have coins that they exchange of fruit that others have picked. Now suppose that everyone is getting old and soon no one will be spry enough to pick fruit. Frank says “I know! Let’s put half our coins in a box so we can use it when we’re older to buy fruit!”. Obviously Frank’s idea won’t work. In the short term fruit will get cheaper because there will be the Sam amount of fruit but less money people can spend on it. Then when everyone gets old there won’t be any fruit.

Private investment works differently because banks invest the money to build tools (ladders) and plant more trees to make fruit picking easier in the future.

So from the perspective of the government guaranteeing a secure income, it makes sense to pay current retirees with current taxes.