Thats because as you said they haven;t been revised since the 70's.Such ashame.I love the old 24k USGS topo's, they are what got me interested in maps when I was just a kid. But they really don't reflect the current reality in many cases. For example, Bulldozed Road should be familiar to most people, it is on all my maps, it's on OpenStreetMap and even on the new USGS National Map. It shows on the aerials back to the 1980's, but it's not on the old 24k topo map.
Devious-Mount Road is another well-known road on current maps like my topo's, OpenStreetMap and the USGS National Map. But it's not on the old 24k Topo's either.
These are just a couple examples, there are plenty of others.