My understanding is that energy efficiency requirements prevent this. Dishwashers have a fixed energy budget they are allowed to expend, which may not be enough to heat cold tap water up to the optimal cleaning temp.

That’s pretty wild, since the energy needed to get it up to temp would still need to be expended on the water heater side. There are no real energy efficiency gains unless they can somehow engineer an effective cold water cleaning.

So instead it pulls in hot water, but not for long enough to purge the line, so all the 140F water it pulls from the water heater sits in pipes till it cools off.