Thermo dynamics, in short.

In long, because adding some heat recovery system to the engine block would mean decreasing the cooling efficiency of that block, thus making the block hotter, and decreasing the efficiency of the engine. Since the engine makes power based on the differential of heat/pressure from the top of the stroke and the bottom of the stroke. If you make the system hotter, then less energy can be extracted per unit of heat produced from burning fuel. Any energy generation from the waste heat of the block would be offset by efficiency losses in the engine it’s self.

Now, most engines don’t actually extract all the energy they could from that differential, which is why turbo chargers are a thing. They use excess heat in the gas exhausted out of the block, expand it to ambient pressure and temperature over a turbine, that turbine then runs a compressor, and that compressor raises the pressure at the air intake. More air entering the engine in the same volume allows for more volume of fuel without having more fuel than oxidizer to burn it, thus increasing the energy density of the charge, increasing the differential in heat between the top and bottom stroke, increasing power and/or efficiency depending on it’s tuning. But that’s not utilizing heat from the engine block, but heat in the exhaust.