Cause it feels like the shit that's being designed is being engineered by pencil pushers, who don't have to hump that weight day in and day out.
We were skinny dudes--certainly not the gym rats a lot of guys are today-- living on c-rats and cigarettes, humping half or more than half our weight in gear in high heat/humidity, over various difficult terrain. We waddled. If you sat down you had to be helped up. Equipment straps made furrows in our shoulders.
I suspect the average load-out for legs is similar today. Joe won't break. His body will pay...but he will rise to the occasion, always does. I can tell you one thing for certain: ergonomically, the gear designed in the past 20-30 years at least fits better and has better weight distribution than all the shit that hung off us.