[eonwe]

Only a part of the interest comes from risk.

One important part is that if I were to give you 10$ in 10 years, it has less value to you than me giving you 10$ now (which is the difference between lending you lending to me and you not lending to me if you were 100% sure I would pay back).

[baursak]

The part that comes from risk in this case is the biggest chunk of the interest though. If we assume that AAA-rated have "no risk".