In APT, arbitrageurs are so powerful in eliminating mispricing because

$1$ They can borrow infinite amount of capital to trade against mispricing. So anyone else cannot have enough power to fight against arbitrageurs.

$2.$ Brokers are willing to lend infinite amount of money to arbitrageurs because arbitrageurs have the knowledge about misprcing, and their arbitrage strategy is risk free.

$3.$ Arbitrageurs do not need to be rich to begin with $($i$.$e$.,$ having a lot of capital$)$ because they can make an arbitrage portfolio that requires zero investment in theory.

$4.$ APT essentially says that if anyone in the society observes some price is wrong due to the unique information they have, they can help eliminate mispricing by constructing a zero investment and zero risk portfolio to eliminate mispricing. This is of course in theory.

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