Sometimes you have other things to do/on your mind. Besides, you are all doing this voluntarily and without an agreement of compensation, right?
At least for me, it is part of the netiquette deal that in return for advice received free of charge in the public consultation, one also feels a solidarity within the community. The feedback, in this sense, belongs to the benefit for readers of a answered question. Unfortunately, quite a few in this sense delinquent questioners are at the same time those who, on the other hand, quickly come back with the next question ;-(