Sadly, what makes toast so great is a chemical reaction known as the Maillard reaction -- it's the same chemical reaction involved in giving grilled red meats their signature "crust," or caramelizing onions. As with both of the other two examples, the conditions at which the Maillard reaction takes place are often pretty restrictive when it comes to time and temperature, and adding more temperature doesn't always increase the rate of the reaction in a controllable fashion, especially when you consider that bread is mostly air and air is a rather impressive insulator.