It's for security! If someone who isn't you is able to sit down at your computer, bypass your OS login, and launch the game without us asking for your game account password, they'll be able to do unspeakable damage to your game's save file! They might even finish the game for you!
I’m no expert in the matter but from what I gather here Steam Keys and in other comments around the web, Valve allows free key “generation” for resale in other stores.
Just one more launcher bro just one more login bro just one more email verification bro just one more app bro just one more website bro just one more link to another game store bro just one more authenticator bro just one more