I feel like this question fundamentally demonstrates a lack of understanding of what arbitrage actually is. The obvious answer is: Because water seeks its own level.
If there is an imbalance in the market, someone will balance it. That's how it works. Let's say you have two different securities, that both represent the same underlying aspect of the economy. This happens all the time; there are multiple ETFs that represent the same basket of stocks, there are options and futures on those ETFs, including different types of futures that represent the same ETF but at different levels of leverage, there are options on those futures, and so on. All of this exists because there is no reason for them not to—people believe they can make money, reduce risk, or whatever by using these derivative instruments, and maybe they can.
But all of these stocks, funds, and derivatives are valued by one thing, and one thing only: their price on the open market. So, what happens when one of these funds get out of whack with the value of its derivatives, or even the underlying stocks themselves? Remember how we had multiple securities representing the same underlying aspect of the economy? Well, one is now worth more than the other, despite the fact that they represent the same thing. There's an imbalance, and if it gets too far out of balance, people are going to realize, wait, I can just buy 500 shares of SPY and short one ES contract, and that's free money! And that's entirely correct, you can. But the result of doing so is a minor market correction; the price of SPY goes up and the price of ES goes down, and boom, we're back in equilibrium.
So, arbitrage is a necessary and unavoidable aspect of the stock market. You can't have a stock market without arbitrage, because the stock market reflects so many different facets and ways to trade ownership and value of companies. Arbitrage is, in essence, the means by which information flows between various aspects of the market, and it connects them together so they effectively move in lockstep. It is nothing less than the market leveling itself, just as water poured into one side of a swimming pool fills every part of the pool.