As you know writing the short bio is very friendly towards other member who want to know a bit about your background and your experience level, and even a recommended thing to do, the feature wasn't done for nothing as all of the site's comfortable features. But as most of the very comfortable and recommended features of the site, it is optional and not required at all. Many of the things here are optional but very recommended but it does not really have to matter for us. Most of the serious members between us do write a short biographies which include a little background and magical experience, but as I said not any member has to do that. There are few reasons that someone might not write a short biography or add a profile picture:
1. The person is not a serious member of this site and he/she is here only to ask questions and get answers from experienced magic practitioners.
2. It might be a fluffy bunny or anyone who has only harmful and bad intentions toward the site.
3. As you know, not all of the people here like talking about their selves or writing short biographies so they let people to draw their own conclusions. No one really blames them, It is optional and every one can choose if to use that or not.
4. Several people have a low English level and it limits them with using few of the site's comfortable features, writing a short biography is one of these features.
5. Few people do not really know how to upload pictures. Yes, this is not funny, not all of us are great with computers. When i started to use my computer I had no idea how to upload photos/profile pictures to any of the sites i knew, so I always remained without a profile picture, but nobody really gave me harsh comments for that or even ever complained. Every one and his own!
6. It also takes some time to choose a profile picture, you don't just upload every random picture you find. Few prefer to upload images that match their belief system and other factors.
7. As you said, many profiles are really abandoned, Lol.
As i presented, there are many reasons that people would not be able/want to upload images to the site or make short biographies.
As you know the bio is not the thing that actually determines if a person contributes good stuff or if he has knowledge and good intentions. You have to draw your conclusions about a person about his knowledge level and other stuff be observing as well, not only by pressing on his name and reading his biography. People do not have to be judged by how their profile looks, they should be judged by their behavior, knowledge and maturity. As i said before: everyone and his free will.
But it is a good point though. The biography / image uploading system is really recommended and comfortable, but people do not have to be judged by that. People should try using it, it is very helpful , but not required as well.