(Corrected the second sentence to how you probably meant it)The point I was making about “named/legendary” items not having mod slots is that they are super rare, unique, and powerful in their own right. They should[n't] be moded because then they become ridiculous.
Many mods currently don't empower the guns (much), they customize them. Someone wants his shotgun to have spread, another one wants to shoot concentrated volleys with maximum reach. Someone might like his AK with precision damage with a semi-automatic mod, another one wants to shoot bursts ...
What would it be like finding a legendary gun and then realising that you don't really want to use it because you can only shoot bursts with it?