I think that people who don't have to think about such things simply don't. I've run into it with men who don't realize that women think about things like, when walking on the street at night, whether or not that man who just crossed the street towards them is a threat or making sure that, while at a party, they never drink something they haven't had complete and constant control of.

I've also seen it with people who don't have disabilities. Most don't look at a building and consider whether or not someone in a wheelchair could open the door or turn that tight corner or get over the decorative doorsill. They don't look at a neighborhood with no sidewalks (or bad sidewalks) and wonder if they'll be able to walk to the bus stop without injuring themselves or getting hit by a car. (I don't normally use a chair or scooter, but I have used a scooter at GenCon and Origins because otherwise I'd be in agony after the first half hour and unable to do anything much after the first half day. It's enlightening.)