No, bonds held by the federal reserve are rolled over. The debt is paid back but it is immediately replaced with new debt of equivalent value, off of the open market. The process is described here: https://www.newyorkfed.org/markets/treasury-rollover-faq

>On the auction settlement date, the maturing Treasury securities are exchanged for the newly issued Treasury securities.

In net, the debt is only paid back when the central bank is actively reducing its treasury holdings, by rolling over less than the total amount of maturing debt. That is basically the reverse process of 'printing money', it takes the money back out of the wider economy.

>Why do you think people buy them?

Certainly 'people' buy them because they intend to get paid back, yes, but the central bank doesn't have the same motives. It increases and decreases the amount of treasuries it holds in order to control the money supply. That's the whole purpose of it holding treasuries.

edit: here's some bedtime reading https://theconversation.com/how-the-federal-reserve-literall...