It could have just as easily been called e.g.
The exe file is just a bootstrap that prepares the engine, and loads the actual mod (like Half-Life 2 or Counter Strike: Source), which in itself is a bunch of data and DLLs. You're never running the game - you're running the engine, telling it to select a given mod (note how the games are run like hl2.exe -game cstrike, for example). Source Engine games (all those you've mentioned) are really closer to what you'd usually call a ('total conversion') mod.
