Pre-rinsing uses way more water than is typically necessary just to get the water hot, especially given that to be an effective pre-rinse you're going to want the water to be hot already before you even start.
You don't rinse everything! There's always a few items that need an extra squirt before they go in. (Like pots and pans.) By the time those few items have an extra squirt, the water in the pipes is hot enough to start the dishwasher.