I've seen many profiles that appear to not be filled out just looking at them in the feed, because they're just a green square with text. But when you click on them, they are filled out to some extent, they just haven't set a picture to be their main profile photo. My hunch is that some users don't know they have to set it themselves, and that their profile may be ignored by more people because of it. Automatically setting the first uploaded picture to be the main profile picture should lead to more engagement.