I think there are a lot of reasons people don't ask. Some people forget, just like their supervisors. Once they move onto the next project, they don't look back to what they've finished. Other people really are intimidated to ask. There are people who probably think if there was anything overly negative or positive to say, then their supervisor would come and tell them. And I'm sure there are at least a few people out there who just are afraid of getting any negative feedback so they avoid it.