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The pilots take off when they take off. If their take-off clearance isn't obviously time limited, they are certainly allowed to take their time. If they rush the take-off they might skip crucial items from their checklists in the haste, we learned that lesson the hard way already. Or what would have happened had the southwest jet had an issue on take off and had to reject? There is a number of reasons to reject a take off before decision speed that will take the plane down the rest of the runway on full brakes.