Hot water from the house supply isn't that hot though? My dishwasher gets MUCH hotter than the hot water supply... and I don't think the heater is "tiny" I think it's a rather substantial element. The dishwasher also doesn't need to heat up a "bulk" amount of water, just the amount of water used for washing the current load of dishes.

Watch the video; it makes a huge difference even though the hot water input is not as hot as the water can get when the dishwasher runs its heating element.

Also the size of the heating element is irrelevant. What matters is the power dissipated. Most dishwashers in the US will use only about 900 watts of power even when plugged into a circuit that supports 1500 watts. In the EU they often hit 3000 watts. Even when just heating up a gallon or two of water that makes a huge difference.

My cheap GE dishwasher uses a hot water line, but also has an internal heating element to "boost" it, and help dry. My electric bill definitely suffers if cold water is used.

Hot water from the house supply isn't that hot though?

Depends on how you have it set. My current and previous hot water heaters had thermostats which permitted adjusting the temperature.

They also had warning labels on them about scalding water. If it's hot enough to scald, it's hot enough.

The dishwasher also doesn't need to heat up a "bulk" amount of water, just the amount of water used for washing the current load of dishes.

If you're washing dishes and someone is, or has recently, taken a shower; or someone is, or has recently, done laundry; or someone is, or has recently shaved or done any of the other dozen things that draw from the hot water heater, then the water is already hot and available and doesn't need to be heated all the way from cold by the dishwasher. A properly insulated hot water heater can retain heat for quite some time.