Because there's no difference between distributing a DRM-free disc and having a GOG key. As long as GOG doesn't provide any additional service than being a retailer, we won't see GOG-keys very often.

Why is the sky blue?

http://spaceplace.nasa.gov/blue-sky/ :p

Because GOG doesn't have any deals like that with external publishers?

Question, does Dragon Commander come with a 16 character gift code (which are valid indefinately) or with an 8 (or is it 10) bonus code, which do have an expiration date?

Because if it were purple, it would be weird.

I'd prefer green, if you asked me.

But grass would have to be red, then.

'I see skies of green,
green-ish roses too,
I see them bloom
for me and you
and I think to myself
why did I drop my screen'

I would change the last line to "I will shoot you in the spleen", dunno why.

While we're in "LSD Mode" I want more colours for people's hair!

BLUE HAIR! GREEN HAIR! PURPLE HAIR! YELLOW HAIR! Oh wait.....

"simple" answer = wavelengths of light and refraction angles, but the site you linked does have a pretty good explanation

As to the OP, I would guess it's because more publishers don't have an agreement or desire to provide GoG keys.

Because gog isn't as big as steam yet. That saying some bundles started adding gog keys.

I can't believe it, from GOG keys were are now at the color of the sky......
And i think that a pink sky would be lovely :p
[url=https://www.youtube.com/watch?v=41M7aB4Tyh8]pink pink

Wouldn't the green light make the red grass appear black or something? Oh no, wait, it doesn't work that way. That would mean that GOG would have to receive a share for every single physical copy of a game sold and the question is: for what? Physical copies can easily provide the biggest benefits of GOG's services (DRM free, bonus material etc.). Cooperating with Steam makes a whole lotta sense in many cases because of all the online services they provide - especially if your game largely depends on online features it probably makes sense to get rid of the responsibility of maintaining servers for a revenue share. Another reason is probably that some publishers may consider Steam a decent DRM (I can't think of any other reason why Silent Hill: Homecoming was a Steam exclusive) or that publishers won't have to deal (as much) with customers complaining about damaged discs since they can get the game from Steam anyway.