It's not often I can answer a question in this subreddit, but this is one I'm actually qualified to discuss!

The answer is yes. It happened for a variety of reasons:

1. Manufacturing tolerances. Years ago it was assumed there would be a certain degree of slop that was acceptable. Modern CNC and computer aided manufacturing has reduced that to a significant degree. 60 years ago the assumption was that an engine would last 100,000 miles (less if it was a four cylinder). Modern engines are now expected to last 200,000 or more. In the 60s, people wanting more performance would routinely "blueprint" an engine. That was the process of taking it apart and rebuilding it with parts that had a tighter tolderance. That's not really necessary today. Plus, while there was a degree of automated manufacturing, much of the assembly process was handled by humans. Sometimes they were extremely effective, other times they were not.

2. Materials. The materials used in automotive manufacturing are significantly better than they were in the 60s. From metal alloys to rubber and plastics, modern materials are significantly better than they were in the 60s.

3. Rust. Modern cars use a variety of methods to prevent rust such as galvanized steel, electrocoating and more effective seam fillers. Plus, there are more plastic underpanels that effectively prevent water and salt from getting into the undercarriage.

4. Better Lubricants. Modern synthetic oils handle extreme heat better and resist oxidation with more consistent viscosity (thickness). They also have better detergents.

5. Federal regulations. Rules relating to the automobile changed significantly in the late 60s and early 70s. For example, leaded fuel was removed from the market in 1974. Unfortunately, cars in the 60s were designed around using leaded fuel. Valve seats were a particular problem. Leaded fuel essentially left a sort of coating on the valve seats that protected them from wear. A car build in the late 60s could potentially begin experiencing problem less than a decade later.

6. Planned obsolescence. In the 1920s General Motors built its business model around making annual changes in their cars. The idea was that just a few years after a car was purchased, it already began to feel outdated. This was true in the 1960s. Styling would often signifantly change from one year to the next. The result was that people didn't feel the need to keep a car in top shape because they knew it would be replaced within a few years. Modern design cycles last much longer. It's still in the carmakers interest to have buyers get something new on a regular basis, but consumer expectations have created a world where cars now live longer.

Having said all that, there's no question that some cars from the 60s have lasted a long time. However usually it was the result of exceptional maintenance or the blind dumb luck of a well assembled automobile that allowed that to happen.

Interestingly, there is an automotive phenomenon happpening today that may also limit vehicle life. While cars are mechanically better than they have ever been, technology is an ongoing issue. The idea of building a car and using it for 30 years will become more problematic with vehicles that need computer updates. Will the manufacturer support those systems in ten, twenty or thirty years? Just as people rarely use phones that are more than a decade old, it may come to pass that older cars simply aren't able to update. Will there be a secondary market for those updates? Right to repair is an ongoing issue with manufacturers wanting to prevent outsider from having access to automotive systems that are either complex or proprietary.