Theres a few reasons. I always thought it was kinda stupid. I think it devalued the siccness. I think most props are empty arbitrary pats on the back for mediocre effort, or just obligatory pay back because person 1 propped person 2. I think most things on the sicc aren't very funny/interesting and usually reposts from other sites. Not like I don't do it myself, but I don't expect any return either. I think a once fertile breeding ground for funny photoshops and roastings, and most importantly, original content, has diminished to timebomb using MS paint to make fun of people. So I guess to cut to the chase, Im not impressed, I'd say less than 1% of crap that gets propped is actually funny. So I basically choose not to participate. Plus it's kinda become my "thing".
That certainly wont be happening. The day I prop 0Ro is the day I retire my Freedom Town satin Jacket.