Why do cars not have electric heat?

Some cars do offer an external engine block heater for colder climates. You actually plug it into a socket like any other household appliance. There may even be aftermarket varieties if you wanted to modify your current car. As for why it isn't on the car in a self-contained unit, you've got to consider that batteries operate much less efficiently when it is cold and sustained use (say, warming up your engine) would likely kill the battery before it would even get warm enough to make a difference. The process involves warming up a giant chunk of metal filled with liquid that is specifically designed to keep the engine cool, and that takes a lot of energy. It is much better to just have the engine run and produce the overabundance of heat combustion provides.