Lots of reasons.
-Taking pictures is time consuming and the results are often unsatisfactory. I do not have a photographer and my house is dark. After a 14 hour work day the last thing I want to do is take pictures of my disheveled self in a dark room. I'm certainly not getting up any earlier to do it either.
-I'm pretty formulaic about my outfits. I have been wearing the same handful of pieces all summer. Not much to show.
-I'm not usually in need of or desiring of feedback. I'm at a point where I trust my instincts. I'm not trying to be arrogant--I still post and ask for feedback on things I am unsure of, but I don't need or want every outfit to be scrutinized.
-A few things I wear are not looked upon kindly by the YLF community (hello brightly colored tights) so I doubt anyone will be drawing inspiration from them anyway.
-Privacy. This forum is full of lurkers. I'm not paranoid or overly protective, but making a regular habit out of posting pictures of myself is taking it too far for me. We all have different boundaries when it comes ot these things.
-I read a lot of the WiW's but don't comment because it is hard enough to catch up with the forum at the end of a day or over the weekend. Often the existing comments are things I would have said anyway. I have to prioritize and the WiWs are on the bottom of the list. Since I do not participate in WiWs, it would be unfair for me to post WiWs and expect others to give me feedback (which as I said, I'm not particularly interested in anyway).
There is plenty of inspiration floating around from the WiW regulars, so I leave it to them to volunteer their time in this instance. I simply choose to volunteer mine in different ways.
Hope that clarifies things.