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.
But they're not significant. The National Insurance Fund is supposed to keep a minimum overfund of £24B (at current spending).
It's now at £79B. It's significantly overfunded.