They obviously wear very well; there used to be one in my old sailing club at Fareham that was in possibly better condition. The owner who had had much bigger cats when he was younger really rated it as a "good little sea boat" and went to France several times in it.
I don't think it's unusual these days. What you're seeing is an improvement in production quality in the late 80s and 90s that made boats last better and made them easier to clean. Despite the "they don't make them like they used to" rhetoric on these forums, older boats were a nightmare and often just cleaning the bilge would get you a hand full of glass fibres. Add to this the idea of maintainability such that fittings can now be accessed and rebedded and replaced easily you get a boat that will stay drier. Less uncovered wood helps too as there's nothing to slowly rot from the side you can't get to for varnishing.
Well despite post #5 there is nothing wrong with the build quality of my 52 year old glass boat. The quality of the cleaning however does leave something to be desired. I do however sail her which is much more fun than cleaning
Exceptions to every rule, of course. I'm sure there were boats made to higher standards in the 70s and earlier, but generally the designs and manufacture methods weren't as geared towards maintenance and cleaning, and you'd expect this stuff to improve over time. Even simple things like electrical conduits which weren't necessary at the time and often later bodged in by owners make a differemce. Any time you drill a hole you risk leaving a core exposed