why series-type? (from a bit of research, seems these are also called EREV (extended-range electric vehicles, in the context of autos) simpler engine since you

why series-type? (from a bit of research, seems these are also called EREV (extended-range electric vehicles, in the context of autos) simpler engine since you can have an electric engine only and not have an ICE engine with more points of failure/maintenance?