The main reason here is the lack of advanced tools to design a multiplayer mod, and then the required budget to work on said mod. Though they cannot do it because of many reasons. There are many players in the modding community who are ready to work on building a Witcher 3 multiplayer mod. Imagine a game like Skyrim that has way more advanced and modder-friendly tools, and si still struggling to bring a multiplayer mode to the table. Now some of you might think that Skyrim did it! Why not Witcher 3? Yes, there is a multiplayer mod for Skyrim, but it isn’t fully functional and isn’t very player-friendly.
So far there is no word on if it is even going to be possible to play this game in multiplayer or make a Witcher 3 multiplayer mod. I know it hurts to say this but I don’t think we can ever share the experience of playing Witcher 3 with someone else. Just to put you out of your misery, let me be blunt.