Steam has no WAW DLC packs to deliver. Everything is in the game from previous updates/patches throughout the game's life (as well as some other games used to do in the past). MW2 was the first COD on PC to do standard console paid DLC packs.
While some game publishers offer free DLC packs, others require gamers to pay. Sometimes, these DLCs are very costly, with some costing as much as the original game.