The reason is that there is no need.
Using petrol to charge the battery is simply wasting energy. Every time energy is changed from one type to another some is lost due to the inefficiencies of the system. There are 4 states of energy with the Ampera - Chemical (battery), Chemical (petrol), electrical and kinetic (speed). Whenever you convert energy from one type to another some is lost.
When running on battery the conversion is from chemical (battery) to electrical to kinetic (speed).
When running on petrol it is from chemical (petrol) to electrical to kinetic.
To charge the battery there would be several extra conversions in the process... chemical (petrol) to electrical to chemical (battery) back to electrical then to kinetic. This is very inefficient and unnecessary as the Ampera can simply use the petrol directly.
In fact, you are wrong to say that the Ampera doesn't charge the battery it is just that there is no "charge" setting controlled by the driver. The system does keep the battery charged to its minimum state all the time. Recovered energy when regen braking will charge the battery. Mountain mode will keep the battery at about 40% charged or charge it up to 40% if it is below 40% when mountain mode is engaged so it does charge the battery then.
Basically, why not use the petrol directly to create electricity the once instead of converting it, storing it and converting it back?
I hope that helps.
