Because maggots can only survive in flesh, the flesh of humans or animals. And, the temperature has to be high enough in the decaying flesh for the eggs to hatch. Some maggots are also species specific...they only inhabit and consume one type of flesh (there are bovine maggots, porcine maggots, etc and "equal opportunity" maggots that don't care.) They don't eat plant material, however.

Bihai, thanks for answering my humble question, it really did puzzle me! :) arum

Kind of on-topic with this thread according to what you want to call a maggot: Small flies hovering around an amorph not flowering is something to be concerned about.
The larva from Drosophila, Orfelia, & Sciara can all hatch & start feeding on the tuber, causing it to rot. This is especially likely if the tuber has been damaged or is too wet for very long and may explain the sudden rotting some growers experience.
Keiko

keiko, thank you, this is how we learn. :) Arum