The disc doesn't have the game files. It just has the game key you bought to verify to Steam that you own it. Then it starts downloading from Steam's servers.
Most games are distributed this way now. It's cheaper for them to market that way, and easier for the customer to verify any missing or corrupt files, or reinstall the game. Plus there's never a need for the customer to keep the game on disc or drive, because all the backup files are on their servers.