That's the issue. I guess they get rid of the real firearms in order to make people use those custom weapons. It could be like Dead Space 3, and most combinations are junk.
The whole weapons crafting really reminds me of borderlands. Same weapon slightly different stats. The melee weapon crafting looks even worse. There are only 2 stats - damage and weight, and each mod gives slightly bigger damage with some benefits like armor piercing from mod to mod. The way I see it they made so many mods with small differences so that the player spend more time scavenging scrap to constantly upgrade his weapons.