It's an American thing. Our dishwashers get hot water from the line, there is a heating element, but it is not very powerful due to our lower output electrical sockets.

I've heard other countries have washers that are powerful enough to heat their own water, which is why they are attached to the cold water supply.

Of all the things. Like what the fuck Germany.

Interesting. Why? Do the dishwashers there require only using cold?

In the US, it is always meant to be connected to the hot tap. The first rinse cycle doesn't heat the water (or doesn't run long enough for the heater to make it hot, anyway). It depends on the hot water from the line to be maximally effective that first rinse. And once drained and refilled for the wash, the water will get up to temp eventually, but it's more effective if it starts hotter.