Why goblins have no milkable creatures? Why not brew kumis? Why not milk sapient creatures?
Because they don't eat or drink.
Goblins not eat and drink for worldgen. Also trolls do eat and drink. Also forgive me but I don't understand what the "brew kumis" part means.
I believe it means "to make fermented dairy product, traditionally made from mare's milk (mildly alcoholic )"
Yes, alcoholic kumis from ogresses milk. Kumis can be made from all milk, not only mare's.
But they don't eat.
Or drink.
You can mod in both if you want to change goblins. But then, you can also mod in milkable ogres too. Toady doesn't need to do this, because it makes no sense. Goblins don't eat or drink (or sell cheese).
In my opinion it still makes less sense that they don't eat or drink than if they did, they already do things they don't need to do to survive, there's no reason the two need to be mutually exclusive. You can have goblins eat and drink but still only die by violence, eating and drinking is something all living creatures like to do. If goblins are to going to feel like a real civilization, as it seems is the point, things like food and farming need to be part of the equation.
Just because they don't have to doesn't mean they shouldn't, they aren't going to die if they don't have craftsmen either, but it makes for boring creatures if they don't do normal things just because they will survive either way. Also goblins eating make them more scary.. eating sentients, or any disgusting raw meats, or ogre milk, or whatever it would only make them feel more real and alive. I think they should act like vampires, immortal but getting ornery, having mood debuffs, increased stress, etc from not eating or drinking.
Edit: Yeah I saw that part about Toady not being interested in them herding animals, but my central point remains the same. They should have the desire and ability to eat regardless. It could be as simple as them ripping apart and eating raw anything that wanders too close or consuming their dead beakdogs, trolls, or whatever. Just like they should steal and consume whatever alcohol they get the chance to steal.