Because where the base game is doesn’t matter. If I get the game on steam or on Epic or GoG doesn’t actually impact my game play all that much. The game exists on my machine.
The only thing that buying the game on steam vs another vendor impacts is where I can buy my DLCs. If I buy the game on GoG, then I am forced to purchase the DLCs on GoG.
But if DLCs are agnostic, and I get the best experience buying the game on GoG, then, when the DLC goes on sale on Steam, then I can still benefit.