They used to. At least when I was in high school that was the rule. The main problem with that is that you get ready to race. BAM! BAM! Someone false starts. Everyone go back to the line. A couple more breaths. Ready to go! Set! BAM! BAM! Someone else false starts.
Go back to the line. You are pissed off now, your timing is off. Ready, set, BAM! BAM! YOU false started. D’oh.
Back to the line, etc.
Meanwhile the fans are in the process of getting their mail forwarded to their seat in the stands before a race finally starts where nobody jumps the gun.