Because it isn't the same as public stock markets. The price on different bookmakers is different because the product is different, bookmakers aren't offering a

Because it isn't the same as public stock markets. The price on different bookmakers is different because the product is different, bookmakers aren't offering an equivalent product to each other (for example, the biggest one is regulation, there is no stock market that invests tens of millions of dollars in stopping people buying stocks, every large bookmaker does this). It doesn't improve efficiency in any way because the price you are taking is known. If you are betting on the open with Asian books, you are adding to price discovery. By the time most bookmakers open their lines, the price of the risk is known so you placing your $100 bet isn't improving efficiency, it is just noise.