I understand that the main reason for not retracting flap as a 'rule' would most likely be because this type of training is being provided to students who cannot be relied upon to manage the decision; so after consideration of the way humans work under pressure there is a 'more reasonable' chance of success with the flaps as they are etc because there is 'less' to do and it will likely lead to a lower controlled touch down speed.
By retracting the flap (large changes in configuration) there is a considerable sink below the current glide path followed by an increase in speed... supposing you survive the extra sink and don't hit anything at your increased speed, you will be able to glide further from where you are when best glide AoA is re-established.
In doing this at low altitude e.g. <500 AGL you risk misjudging the extra sink as this procedure would only be done with an optimistic view of a worsening situation; and you also risk the higher possibility of entering a spin or a stall and dying for sure.
At higher altitudes or with small changes in flap (e.g. 40 degrees up to 30 degrees on a C172) and with more experience your chances of success with this procedure are much better...
