Yes I think many drivers are half asleep!! One day I will try drag racing and I'd be very interested to know my reaction time. It also helps if you have good eye/hand/foot co-ordination! If you intend to enthusiastically leave a green light you certainly need to have you wits about you; not observing cars in the opposing cross street means you could potentially get cleaned up by a red-light runner....
There are other techniques involved too.... If you stall it up too early, the other driver gets a heads up that it won't be your average green light! Plus, if you hold higher revs for too long with your foot on the brake, you can overheat your engine and/or transmission.... Break traction at the rears with an LSD final drive and you could slide sideways without any forward movement and potentially hit the car next to you (ugly)!
Well this is what I've heard anyway....
I know of other people that've done all sorts of weird things in the past. Like installing a rear brake-line lock with brake light cutout. He could hold the car with the front brakes only, to enable huge burnouts without damaging the rear brakes. To everyone outside the car it looked like he wasn't using the brakes at all and they'd naturally assume he had HUGE power!! Sneaky.... Of course that was his drag car!
BTW I'd never advocate road-racing; there's just too many risks.... Those that do it with other people (and even kids) in the car should be locked up.