Hello,
I agree with you Piff Paff. In fact the Boeing FCTM specifically mentions it as such; In case of engine failure during go-around, treat it as a flaps 15 takeoff.
The reasons could be numerous but one which I can think of is the speed. The Vref for 2 engine approach is either Vref flaps 30 or 40 (this is B737) + 5. If one is on single engine approach, the Vref is Vref 15 + 5. The difference in speed is quite significant. In a single engine go around, the higher speed is used to compensate the loss of lift through the flap retraction. On a two engine go-around, if one loses an engine, it may be that he has not reached the 160kts ias the flight director is asking him to maintain with flaps 15. So, if flaps is further retracted to flaps 1, the resultant decrease in lift will be, I believe, significant. You may not be able to even achieve a positive climb gradient, let alone the required climb gradient.
I hope this helps. As usual, I stand corrected...