I think it's also because EREVs need a performant generator that isn't much different from the drive motor. So you'll have a full ICE+EV drivetrain on top of an EV as its battery for it to work, which is not ideal.
Compared to that, other hybrid systems like the motor-clutch combo packs that sit between the ICE and gearbox, as well as the fixed gear Toyota systems, can have motors that are weaker, lighter, and cheaper than the ICE, while being able to be additive to the ICE power output. That makes more efficient use of steel and copper than full serial setups.
BMW i3 had such EREV option, but it apparently used a cheap OEM bike engine(not like a BMW Motorrad engine) and also could not overcome power draw of the EV drive motor. So it literally just extended the range, not allowing it to run indefinitely at highway speeds with just refueling. Nissan e-POWER system is powerful enough to do that, but then the readers of comment will have to back to the line 3.