We should have to make builds around our weapons to make them worth using... That's the whole point of the builds

No it isn't. The point of a build is to spec into a particular playstyle, not to make something merely usable. The Seventh Seraph weapons are TERRIBLE without Warmind Cell mods. They should at least be average. The Warmind Cell mod system would be great if they made already decent weapons great. Instead they make terrible weapons decent.

why do you get upset that you can't use 6 more weapons without a certain build

Because I like chasing loot. That's the entire point of the game. I like grinding for god rolls. I like the pursuit of good items. Seventh Seraph weapons are not good items.