I remember being surprised when I first read the results of a survey on extra credit published some years ago in Teaching of Psychology. Almost 20% of the 145 faculty (across disciplines) reported that they never offered extra credit and another 50% said they offered it only under exceptional circumstances. The two most common reasons for not giving extra credit were that it encouraged lax, irresponsible student attitudes and it was unfair to offer it to select students (say those doing poorly). I also think it is avoided because it means more work for faculty and most of us already have more of that than we can handle.