In a typical hybrid car (that's not a plug-in hybrid), the batteries get charged from the engine running and from regenerative braking. They then power an electric motor to assist with locomotion. You might park your car in the garage with the batteries at 90% capacity, or 70%, or 50%, or whatever.

When you pull back out, the batteries are then still at that "whatever" level. Doesn't really matter. So how does the car's battery bank know or care whether it was topped off via shore power while parked in the garage? Why is having an onboard charger for shore power (or a plug for a wall charger) such a difficult engineering feat? Seems like it would just require the addition of a part that costs a few hundred bucks.

RVs and boats have a "house" battery bank that generally functions this way. Maybe it's getting charged by the alternator, maybe by solar panels, maybe by shore power. But hybrid cars having a shore power option (i.e. PHEV) seems like a real premium option that is not easily built.

What am I missing? Why does the battery bank care where it got its charge from?