Because it's not an actual investment and can't run out. Like US Social Security and many other national schemes, the UK is pay-as-you-go. Money coming in is immediately paid out.

Any funds lying around are supposed to be for temporary imbalances, but became significant due to a major demographic imbalance: the Baby Boom.

You can buy i-bonds in the US. You are limited in how much though. They are pegged above CPI. You never hear about them because no one makes money on it. And maybe it isn't that great of an investment.