More times than not, people posting pictures and asking for critiques don't want to hear the truth. They are fishing for compliments regardless of their level of talent. They have been told by friends of family that their art is good, and they are seeking confirmation for more neutral sources.
I find that the skill level of most are amateurish at best. Often times I have to ask myself, did this person even put in any effort or did they just throw something on paper just to have something to post. I don't often offer any critiques unless the picture shows some genuine ability and effort on the artist's part. I feel my honest critiques of the others would be a little too honest and therefore just prefer not to say anything at all.
I am not by any means saying I'm great. I've been drawing, painting, and dabbling in other mediums for a very long time and to this day I am constantly learning new things and growing as an "artist" and it pisses me off to see some people's half-assed approach towards art. But that's just me.