Why has the expansionary monetary policy not driven down the required return on equity?

In OECD countries, there is a very wide and growing gap between the required return on equity for shareholders (regardless of how it is calculated) and long-term interest rates on risk-free bonds. The existence of this gap has very significant consequences for the effectiveness of monetary policy and for companies' decisions. But what accounts for this gap? There are four possible explanations: A rise in the risk premium associated with corporate risk, due to a rise in actual corporate risk or in the perception of this risk ; Weak investment and therefore weak capital due to financial constraints (difficulty in borrowing, low earnings) or monopoly behaviours, leading to an increase in the marginal productivity of capital; A financial anomaly (due to regulations, financial repression), which means that investors and savers do not arbitrage between risk-free bonds and assets representing corporate capital; The fact that central banks buy risk-free bonds and no (or almost no) equities, which creates a price distortion.