Many people are shocked that computers/systems for 20 years still run, but is says a few things:
1. That people are used to crap code that can't keep running.
2. That people are used to crap products that can't last for more than a couple of years.
If it ain't broke, why fix it? They sent man to the moon on less CPU horsepower than my Nexus 6. Voyager has been running for more than 35 years in the harshness of space.