With the loss of the first three Fallout games on GOG because of legal problems, it made me wonder about the fate of those games to purchase on GOG. This led me to wonder about something else that had been bothering me for a long time: why aren't any other properties owned by ZeniMax on there? Why aren't the older Elder Scrolls games on there? Why aren't id games like Doom, Quake and Wolfenstein on there? Those games fit that website, they're what I think of when good old games (even if that's not the moniker anymore).

I can see ton of reasons why ZeniMax wouldn't (just like I can see why Disney wouldn't put up the old Lucas arts games), but is there anywhere I can find the given reason for why they aren't from them, GOG or somebody else?

Presumably because Zenimax/Bethesda haven't approached CDPR/GOG about listing their catalogue on there. Also it might be trickier going forward since they've been moving into Steamworks for their newer titles with New Vegas and Skyrim.