There's two facets to this, marketing and engineering. Marketing will slap whatever the biggest number they can get away with and not get sued for false advertising or when someone breaks their truck down the road.Modern truck frames are stronger than those of decades ago; no argument there. What I question is the maximum ratings being used today, and the factor of safety applied to today's frames when compared to older ones when it comes to towing and cargo loads. In the past, I think it may have been possible to approach or exceed the OEM limits without dire consequences. Today, OEMs are able to wring more strength out of designs of less weight, but those new designs may be less forgiving when pushed to - or beyond - the rated limit.
OTOH engineering, testing and manufacturing have improved so much that the numbers are realistic maximums where not even a decade ago, forget the 1970s or 1980s, when things had to be overbuilt because there was so much variance and tolerance. To guarantee a 2,000 lbs payload would be remotely close they'd have to build something that now would be called a F450 with modern FMEA capability.
But the flip side is there is no margin and owners have to get used to this. I'm no different. I had a 1991 Toyota pickup that was way stronger than a compact truck had to be so I got used to being able to overload it. The Tacoma now, when they say 5,350 lbs GVWR they don't mean somewhere within 1,000 lbs of GVWR like it used to be.