Have fun playing Siptah with no game engine, no character models/progression, no crafting, no items, no weapons, most of the terrain assets missing etc.
Siptah is only 20GB, because it re-uses almost everything from the base game, and just adds a few extra bits on top. The base game is just over 100GB because it includes all of the assets for all of the dlcs, including Siptah, because all of them are shared - and you need all of those asset files or you won’t be able to see or interact with any of those things if they were in use on a server - if someone builds a building with Khitan walls, you wouldn’t be able to see it, you would pass through like a ghost.
In order for Siptah to be a functional standalone game, it would need to contain all of the assets and base code that are required for Siptah - which would mean more or less all of the assets and base code. In other words, a standalone version of Siptah would come in at around 100GB. If you were to create a special version that bans all other DLCs, you could maybe shave that down to 70-80GB. Plus the price would have to be considerably higher, as now it would have to cover whatever licensing is used to pay for Unreal Engine (since that is included as part of the ‘base game’), and the perceived value of all the assets you would be getting, and the time and work required to ‘cut away’ the few little bits of the Exiled Lands that could be removed without breaking everything.