After receiving clearance to takeoff from runway 23L, the captain ordered F/O to perform Before Takeoff Checklist and confirm runway 23L Final Clear, then execute time set and make right turn to line up using tiller and then transfer control to F/O. F/O did not confirm on flap 20 Set from the checklist due to read back of the takeoff clearance to the tower as he performed the Before Takeoff Checklist.F/O pushed the TOGA switch when he saw the thrust was approaching 1.1 EPR but Auto Trust did not operate, so the captain advanced the thrust by manual. At this moment, the warning horn sounded and F/O called we’d better Reject Takeoff but the captain decided to continue takeoff based on his misjudgment of lightweight(633,280lbs) and sufficient runway. The captain pulled down the flap lever and took the control and continued to takeoff. The aircraft lifted off with flap 4.9, flap 10 when passing 35 ft, flap 20 when passing 156 ft, which led the aircraft to takeoff without the required flap setting. Warning horn sounded for 22 seconds until V1, 128 kts during takeoff and the captain realized Less Flap when rotated and increased speed quickly by maintaining less pitch(less than 10 degrees).4. Why didn’t the captain abort the takeoff?