GOG is probably looking for true believers in DRM. I'm not paying 10 euros for Fallout 4 when it's half that price on Steam. That's just stupid.

Not the Fallout 4: Game of the Year Edition though, only the vanilla version without DLC. Otherwise that would be 10 € on Steam aswell.

The problem is GOG is not allowing you to buy the standard edition for 4.99

That's not what the OP said. It was phrased as if GOG is just charging more money than Steam. I did look it up myself on Steam because i was curious if GOG would indeed charge that much more for the same amount of content.
The topic could've been called: 'Why no FO4 standard edition on GOG?'

I didn't say or suggest the OP said anything. I was just commenting that GOG only gives you the more expensive option. As for GOG charging more than Steam, it's been much cheaper to buy games on Steam for quite a while. I closed my account on Steam quite a while ago, but I can still see that Steam is cheaper. I guess it was inevitable that GOG would be more more expensive with no DRM.

If you buy the full game, its the same price.

Who buys old games without the DLC at this point? If you look into modding you will often find mods expect the DLC at this point in a games life. I haven't checked FO4, but that is the case for FO3 and FONV.

I bought FO4 from GoG, and I wouldn't have bought only the base game, even if available.