Could it be because you would then have one procedure for hand flown G/A and you would have to have another for a G/A off a fully coupled approach? I'm not sure your method would have the right consequences at 50' on a low vis approach.
I much prefer the Airbus solution where you just whack the levers forward as fast as you like. If you have flaps out then as soon as the levers hit the end stop then TOGA mode is engaged.