Because that's not how bread and water work.

If you want toast, you wouldn't use a blowtorch. Intense heat applied briefly is going to burn the bread long before the heat transfers from the outer layer to the inner ones. Too low a temp, and the bread won't actually toast, just get warm and eventually dry.

Same deal with water. You have to maintain a temperature so that it can conduct to the rest of it.

Basically, toasters and kettles are already the Goldilocks thermal solution for those specific materials. So unless you wanna invent self-heating bread or highly thermally conductive water, you gotta be happy with what we've got.