Time is critical here. If you consider the 'standard' 737 and 320 family of machines, with 140/190 seats, the time from start of t/o roll to rotation is between 30 and 40 seconds, depdent on all the the usual factors. A heavy 747 and 340 family machine will take up to 50/55 seconds. These are typical at sea level and with normal temperature conditions.
If, say, Flap 15 is required the time for them to travel from 0 to 15 can easily be 12 seconds. Thus, they would have to start to deploy just as the roll starts, with no real gain in the time that they would be closed.
The reason for the slow deploy time is that they, typically, use a threaded screw-drive. This is able to stand against the enormous air-pressure when the flaps are set on descent. If straight hydraulics were used, the rams would have to be very large. The screw-drives used for flaps (and some other surfaces) are small and can be slotted into the wings (and fairings), they are powerful but slow to operate.