Ahh, there's the rub.

Our national insurance (pension) contributions don't get put away to pay for OUR pension, they get used to pay the pensions of those who have already retired.
Our pensions will have to be paid for using the payments from the (shrinking) number of people who are working and paying when we are retired.

Which is why the government want us all to provide our own pensions, (except that every now and then they decide that we've saved up too much money and they raid the pension funds!!!)