Costs. Complexity. You have a large battery, a gasoline engine with a transmission and 2+ electric engines in a PHEV. And they all need maintenance in the long run.
This will be hopefully mitigated when all PHEVs only run the gas engine as a generator and get rid of the transmission, but I don't think we're there yet.
> This will be hopefully mitigated when all PHEVs only run the gas engine as a generator and get rid of the transmission, but I don't think we're there yet.
If it's cheaper then they'll do it.
More generally, you can remove a chunk of the transmission in a hybrid and use a simpler engine even if it's not a generator. The complexity isn't a big deal. And the moderate cost gets offset by significant fuel savings.
This will be hopefully mitigated when all PHEVs only run the gas engine as a generator and get rid of the transmission, but I don't think we're there yet.