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The reason they don't do that is it isn't nearly as efficient for this use case. When you have a power requirement that has to move hundreds/thousands of tons a very long way at a pretty constant speed (like a freight train) - it makes a ton of sense to have a huge motor that runs efficiently at a constant speed. Especially when the weight really doesn't matter compared to the load, so the diesel engine can be huge, the electric motor can be huge, and any battery systems needed can be huge. When you instead have a power requirement that varies from very little (idling/slow movement in traffic) to very high (fast acceleration to highway speed, constant higher speeds), it doesn't make sense to size the engine and run the engine at any constant speed. It needs to be efficient over its entire duty cycle. And to do that in a car, you have a motor that can run at low revs low power, high revs high power, etc. In a hybrid car, that reality doesn't change. You need the gas engine to be powerful enough to provide enough electricity to move the car quickly. And you need it to be efficient enough to run it at less power, when less electricity is needed. Only scaling it to run at full power at all times or off turns out to be much less efficient.
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