This is a good question with a simple answer: it was designed and built to last longer.
We need to start this brief discussion by acknowledging the shift in product design life and public expectations around this. Back in the day, people saved and bought hi-fi gear, with a view to keeping it for a long time. Manufacturers built gear to align with this general philosophy.
The longer life of older equipment generally comes down to better mechanical design, use of higher quality switches, longer-lasting through-hole capacitors, greater serviceability through component level board repairability and so on. I often see equipment from 1970 with a full set of electrically perfect Elna electrolytic capacitors. Fifty years from what are nominally 2000 hour-rated parts is extraordinary, and yet this is common.
Modern capacitors found in affordable new equipment rarely last this long. Good modern capacitors are excellent, but you need to spend a lot to get an amplifier filled with good Nichicon, Nippon Chemi-Con or Panasonic capacitors. Add to this the through-hole nature of older caps and other parts, which were designed to be serviceable.
Then there is the physical build quality. Older gear tended to use less plastic, heavier grade metal, metal switches and so on. These parts tend to be serviceable. If metal bends, for example, it can often be bent back. Plastic breaks and becomes brittle with age.
The simple fact is that older gear from the ’60s, ’70s and ’80s was designed with serviceability in mind. Production values and the way we view our equipment have changed. Modern gear is often designed to be thrown away rather than serviced when it fails, so it often cannot be viably kept running.
Home cinema equipment and modern TVs are classic examples of this. Ever wonder where all the old TV repairers went? It’s not that modern TVs don’t fail, they do, but try taking your 65-inch TV anywhere if it breaks.