This is because your body doesn’t run on a short-term fuel supply. Your glycogen (muscle energy, if you will) is filled up anywhere between 350 to 500 grams of carbohydrates. If those numbers don’t mean much to you, that’s more than enough fuel to get you through your weigh workout; and plenty for most endurance sessions, too.
Your goal is to promote muscle protein synthesis (muscle growth and repair), and for that to happen, you don’t need a massive insulin spike. In fact, research has shown that a moderate amount of protein and carbs (or even protein alone, more on that soon) can max out the muscle protein response after exercise.