In these markets, there are no informational asymmetries or liquidity and credit constraints affecting the behaviour of economic agents, which is not the case in the real world. And movements in asset prices, that in reality can be affected by liquidity conditions, do not affect directly or via wealth affects spending decisions.
From this perspective, non-standard measures are a complement, rather than a substitute, of standard interest rate decisions: they help ensure that standard policy produces its intended effects, rather than aiming to impart additional relaxation to the overall monetary policy stance.