As an EE PhD, I know for a fact it's not 24/7 and only for a limited lifespan.
This discussion is getting idiotic. Let's just agree that the OP's original question about why cards used for mining have high mileage are because of high mileage.
The science is that the wires/devices in these cards do wear down as electric currents run through them, in a way like water erodes whatever they're flowing through. After a certain amount of use, some connection somewhere will break. When that happens, things stop working as they're designed to.
This is a good point, everything should work UNTIL you start overclocking, in which case there's probably not enough margin in the PSU capacity. At that point something might croak
this book is the best place to start:
Computer Architecture, Fifth Edition: A Quantitative Approach (The Morgan Kaufmann Series in Computer Architecture and Design): David A. Patterson: 9780123838728: Amazon.com: Books
and this is a good resource for how industry goes about it...