Because the return on a bond is not just based on the expected risk, but also on the time value of money (generally we prefer consumption now rather than in the future).
I don't think that's the only source of time value of money.
For example, I'm fairly certain I can buy a car for the same price a year from now, but I am willing to pay a huge premium to have the car now so I don't have to ride the bus for two hours a day while I save up the cash to pay for it.
That is a preference for present consumption over future consumption that has nothing to do with inflation.