Why does a fall in risk-free interest rates inevitably go hand-in-hand with a rise in the risk premium?

In OECD countries, the risk-free interest rate keeps falling and will remain very low as a result of the expansionary monetary policies. But returns on risky assets must remain linked to the marginal productivity of capital, which does not vary in the short term. If risk-free returns are very low and returns on risky assets are stable, risk premia will inevitably rise, which must be confirmed by a greater variability of risky asset prices.