The hot chicken broth helps keep the cooking consistent. If you add room temperature (or worse, cold) broth to the rice, it slows the cooking process and can even cause the rice grains to become mushy. Your risotto won't cook as evenly as it ought to, and most likely will take you longer than the 30 minutes promised to make. It's not impossible to make with room temperature broth (please don't use cold) but you're less likely to have the flawless, creamy, and relatively quick results.