One potential way of increasing the ability of arbitrageurs to correct market inefficiencies might involve restrictions on the movement of investors' money. For example, there are some investment products that do not allow investors to take their money out of an investment at once. They have to take it out slowly over time, like 10% each year for ten years.

Why might these investor restrictions improve the efficiency of markets?

Even though such investments exist, they are are not that many of them. If they can improve market efficiency, why don't investors like them?