There are so many damn school zone signs because you're very, very easily distracted
You forget about signs you've passed as soon as something else distracts you. For example, let's say you're in a school zone. You saw the flashing lights, slowed to 20mph, and then, mid-school zone, you caught a red light. Once the light turns green, if there's not a second sign to remind you, you'll drive off without a care in the world -- until there's a cop with a radar gun. Statistically speaking, traffic as a whole speeds by roughly one mile per hour in a normal school zone, and over five miles per hour in a school zone after a red light.