If you run it correctly, with the hot water at the tao, the first cycle is only a few minutes long and the box is insulated, so there is very little heat loss. It drains all that water out after a few minutes so that all that grease and debris isn't being sprayed all over your dishes that it is trying to wash. There is no reason to add more heat to that cycle, and the heat added would be minimal since the cycle doesn't last long. The next cycle, the detergent cycle is much longer, so the water will lose heat over the duration of the cycle if not heated. That is what the heating coil is used for, to maintain the heat of the, ideally, already hot water.

Why not use the heating element one the first cycle? Energy efficiency, runtime, and equipment cost/complexity. It is a waste of energy to heat cold water when you should already have a tank full of heated water somewhere in your house with a line connected to the dishwasher. But not only that, heating water takes a considerable amount of time. To heat a gallon of water by 80 degrees Fahrenheit (average cold tap is 60 degrees, vs 140 in water heater) with a typical heating element in a dishwasher, it would take just under 15 minutes of continuous heating to get it to temp, and you would need to do that before you started cleaning if you want it to matter. And every cycle after that will need to heat the water from cold too. With 4 cycles to a normal wash (if I'm not mistaken), that's an extra hour to every load of dishes. Then on top of that, you need a thermostat that's currently unnecessary, to let the dishwasher know when it's reached temp. The temperature sensor that is currently in your dishwasher is dedicated overheat sensor to make sure the system doesn't get too hot and become a safety hazard. It's a simple kill switch, too simple to serve both purposes. So you would need both sensors, not just the one, or a more complicated and expensive sensor.

It's not like they couldn't just use the heating element on the initial rinse. They could. But there's no good reason to add extra time, sensors and power usage on an appliance when you already have an appliance that's already done all of that for you. You just have to clear the line of the unheated water. It will save to time and money.