The motherboard isn't expected to be normally unplugged. The voltage wouldn't build up with the motherboard connected, instead the capacitors, filter capacitors, are discharged thru the motherboard. No buildup.
Now you are just going round and round in circles. A miner isn't normally unplugged either. However both a routinely are connected and disconnected over the lifecycle of the device and a powersupply needs to handle that. It is the sole purpose of a regulated power supply. It is what they get paid to do.
If the scenario that disconnecting and reconnecting a mining board would allow overvoltage to occur the same would happen when disconnecting and connecting any powered device. People do upgrade computers you know. If the bogus scenario you believe was true then a high portion of motherboard upgrades would result in destroyed motherboards. Of course that doesn't happen because power supplies are designed to regulate the voltage. The power supply has both a power-on and power-good signal. I doesn't energize the output rails until it verifies the voltage is regulated. There never is a scenario where a power supply lacks an active circuit. It has control of the input power and the ability to power internal circuits for the sole purpose of continually monitoring and regulating the power output.