Having an ICE that drives a generator (alternator?) to power a traction motor(s), without being mechanically linked to the drivetrain, is how diesel locomotives operate. I believe the concept also has been (is being?) explored for linehaul trucks. But I'm not sure what constraints there are on passenger vehicles though... I'm also curious.

If I had to guess, I would bet that the constraints are more commercial than physical... hybrids are already very efficient, so the market for such a vehicle would probably not justify the engineering costs. But that's just a guess!