Now its rare to see a car on the road anything close to that.
You need to visit my area.

There's more than enough of what you described.
Granted cars don't rot out like they used to,
Ummmm.....yeah, they do. I visited an outfit that was selling "take-off" southern pickup boxes, and noticed that the mfgr's don't even paint the front outside wall, or the underside anymore. Just leave it in primer, which is porous, and it rusts out in short order.
and engines are built to last longer.
I disagree. It's not so much that engines are built any different. It's that we have soooo much better ignition systems, fuel management, and light-years improved lubricants that engines don't wear the way they used to, oils don't get contaminated by unburned fuels, and they stay in tune much better.
Seems the majority of people that own a truck anymore just use it for a daily commuter. I see guys with big diesel pickups that look like they haven't hauled anything except groceries.
Here we're in agreement, and it's likely that they haven't hauled anything to any great extreme. I suspect it's due to economic reasons. Don't use it as a working vehicle, and the trade-in value stays up. I suspect a fair number of them also trade in every 2-3 years for those same economic reasons.
Just my opine.....Roger