Because if you have liquid forward markets in bonds, then there's a set of arbitrage relations between the overnight rate, the expected volatility of the overnight rate, and the short-maturity end of the yield curve. And the CB fixes the overnight rate.

Empirically, this breaks down more and more the farther you go from the overnight rate, for various reasons that I will not pretend to fully understand (but which are presumably related to the absence of a consensus on long-term policy rate volatility coupled with credit constraints on potential arbitrageurs preventing the actual construction of the hypothetical tracking portfolios which could enforce the relationship). But it holds up quite well for the short-maturity end.

- Jake

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