Also, there just isn't that much more you can do with an ICE engine - it's burning fuel to raise the temperature to generate overpressure which pushed pistons that turn "pedals", which then act through a complicated set of gears and linkages. Whatever you do, it's never going to be super efficient, most of the energy is lost to heating the coolant. And it's a technology that lots of people have already spent a lot of effort on optimizing, meaning most out the easy gains are long gone. Also, it needs to be fairly light and cheap to fit in a car, and work at a wide range of RPMs, torques, power levels, and it has to be reliable and not too complex.