The primary reason money managers say day trading is impossible, sans the obvious moral hazard, is due to economies of scale against daily floats. Retail traders have an enormous advantage managing accounts up to 5-10 million in trading relative to the viable churn of equities and their respective daily volume. If your managing 100 million, at scale, you have to hold longer and are more susceptible to market conditions and overall index performance. With less money you can invariably move within moving averages, price action and volatility more easily.
