I think MTB's have wider handlebars because riders use them as leverage when climbing and navigating difficult terrain. The wider the bar, the greater the leverage. At least that is what I've concluded after researching handlebars for my own hybrid. Road riders are trying to minimize their aerodynamic profile for greater efficiency. Different bars for different goals.

I tried mounting bar-ends inboard of my grips and controls and it didn't work. I was basically trying to put aero bars on an MTB and it just didn't work well. The controls got in the way and the "Feel" was just awful. I ended up just taking the bar-ends off.

If I reach a point where I'm riding fast enough and far enough to worry about efficiency, I'll do a drop-bar conversion or just get a new bike.