It's because FO4 isn't that beloved a modding landscape, mainly being inhabited by equal parts "photorealistic tacticool gun enthusiasts", "tight-clothes, pose gooners", and "people that like -- and want more -- Fallout 4".
It's because expecting that is hoping to siphon a percentage (people that want to install a whole other game on top of their game to mod/play mods on) of a percentage (people that mod video games at all) of gamers.
It's a similar reason to why Enderal, a fantastic RPG that completely provides a much richer canvas to mod on in terms of "immediate" story, has barely any mods beyond visual replacers, compared to Skyrim. Expecting, and not idly hoping for, deep modding support was unrealistic.