How come not all of the COD 4 default maps appear?

Question

Why don't all of my default maps for call of duty 4 appear? i've already beaten campaign.

the maps i have are:

- Backlot

- Bog

- Countdown

- Crash

- Crossfire

- Shipment

- Showdown

- Vacant

- Wet Work

I'm missing:

- Ambush

- Bloc

- District

- Downpour

- Overgrown

- Pipeline

- Strike

Answer 1

for some reason only certain maps are availabe for offline play. It doesnt make any sense to me but that is the way it is.

Answer 2

I bet your trying to play split screen when you play split screen you dont have those maps