I think it's partly because many of us have no need to know what our arm measurement are. We all know our height, most people know their weight, our waist is important for finding pants that fit, and the chest doesn't fluctuate a huge amount--and is a standard measurement for finding a suit jacket or the like. Arms, though? I've never needed to know how big my arms are, so that isn't a number I can rattle off off the top of my head. Anything that's going to take work to obtain is less likely to be filled out. If the profile asked for calf measurements I bet those would consistently be left blank too.