Our freighters have the mass / balance sensing struts and it is a great backup to the load-sheet numbers. Our procedures say that we have to re-check the pallets if we are more than 3% off the computed TOW or CoG. This is rare, but occasionally happens. It is usually a ramp issue (not very level), I have never seen it be the loading.
I would imagine that with passengers the airlines don't want to scrap the standard weights because they would loose out on revenue. I ask you, how many American adult males weigh 180 lbs?
Obviously, a weight sensing system would have to go hand in hand with accurate loading (ie, weighing passengers) which is easily done - the technology is used for bags when it comes to making passengers pay for extra luggage weights.