I always wondered why "electric with an onboard generator" didn't take off as a subset of available vehicles. AFAIK, hybrids all work with complex gear boxes to allow the engine to directly power the wheels, while also charging the batteries.

I suspect the answer is because it's more efficient from a MPG perspective to allow the engine to directly power the wheels, at the cost of the extra complexity of gearing. Converting motion to electricity back to motion is lossy, and even more so if you insert battery storage as an additional step.