In my opinion, it's because it's the probably one of the first things MOST people notice in a photo. Almost any camera can produce a sharp photo when used properly... even an iphone camera could do it. My guess is because of that, people expect photos from "nice" cameras and lenses to be sharp. A lot of people don't know how to appreciate composition, contrast, and light (not because they're ignorant, but because maybe they weren't exposed to those elements... the general public might not know too much about art), so they look for staple qualities of a "good" photo, such as bokeh, super sharpness, vibrant colours, etc. People want to see what their point and shoot/phone cameras can't do. If everyone can do sharp photos, then the standard for "pro camera" sharp is raised.
Why people obsess over it? Maybe because it's the norm now? Maybe it's because people want to appeal to the public? I don't know about others... but if I'm paid to shoot an event or shooting for my school's yearbook... I'm always shooting my fanciest lenses wide open. I'm always producing photos that people will go "wow" at, photos with tack sharp subjects and bokeh-filled backgrounds. Of course, I'm still keeping in mind composition, light, contrast, movement... but most of my clients and the students at school don't care much about that. I have to do what I'm paid for, that is, deliver photos that will please the general public.
Might've gone off on a tangent there, but the point is, sharpness is what people expect from "nice" cameras and lenses. They might obsess over it because it's what their viewers want, or because it's what they want themselves.
When I'm shooting for my personal pleasure, however, that's another story. Sharpness becomes less of an issue. Blurryness matters little to me, compared to lighting and composition.
just my two cents!