Perhaps American dishwashers are different, but EU dishwashers have 5 independent safety mechanisms on the heater...
1. It will only power up if the water level sensor detects water above the level of the heater. In many machines this is a hardware switch, not software+relay.
2. Power to the heater goes through the door switch. With the door closed, a fire in the wash compartment will run out of oxygen pretty quick.
3. The heater has a temperature sensor on, and software control to turn the heater off at a set temperature.
4. The heater has a thermal fuse, which over 150 C will permanently cut out the heater.
5. The heater isn't exposed on the bottom of the dishwasher where things could touch it - instead it's under a grille or embedded in a pipe that water flows along during washing.
I know all this because my dishwasher caught fire... But it wasn't the heater but a power supply wire to the heater which had a corroded connector. Turns out many years of vibration and steam is hard to design connectors for...
I had a dishwasher that probably had a bad valve that caused it to leak water all over my kitchen floor while I was on vacation. Needed to replace the hardwood floor because it swelled up. (Ironically, my home insurance paid to replace everything except the dishwasher. Sigh...)
This is actually because the compressor can fail, generating heat and so possibly catching fire.
Wires can also melt.
I've seen far more than my share of appliances-turned-bonfires. Shorted wires, terminals, animal fur. Magnetrons and capacitors running amok. Those are especially fun when the microwave is mounted into wooden cabinets. Dryer belt pulleys freezing up causing friction rubber fires.
The dangers abound, they just are never thought of until it happens.
Now you have the extra fun of ICUs being on backorder 6+months (9/10 failures on all new models are ICU/touchscreen board failures).
I'd really hate to be an appliance technician right now.
In my case the dishwasher's program cycle got stuck and it kept heating up the water till it had all evaporated.
I would usually nudge it along when it got stuck, but that time I had been out and forgot about it. I would have never expected that to happen but in hindsight I feel pretty stupid for not replacing the dishwasher immediately when it started acting up.
To be honest, I'm not sure. I can say for a long time, refrigerators were very dangerous and would often kill people. It's why Einstein co-patented a new mechanism for refrigeration.
Likewise, maybe there's something particularly challenging about designing heating elements to work in a watery environment that makes dishwashers subject to failure.
It also doesn't have the same global popularity as more common appliances like a dryer, so perhaps the failure rates aren't as well known, except among the affluent and restaurant owners.
