Nobody thinks that it’s impossible, which is incredibly rare, but rather that it’s very costly not to comply, which is the source of every monopolist’s power. Could Pepsi refuse to sell at Walmart to avoid the huge wholesale discounts they demand over smaller stores? Sure, but it would shoot themselves in the foot, and that’s the source of Walmart’s anticompetitive power, which coerces Pepsi (and lots of other suppliers) and hurts lots of smaller businesses who don’t get the same discount.