When handheld weapons were first coming into the world they were handcrafted, each unique, and thus if you wanted to make any modification to your weapon you had to take it to a dedicated gunsmith to actually physically modify the components of that weapon. In today's world, handheld weapons are largely modular, you can swap components so long as they use a common mounting system and you largely don't have a reason to go to a gunsmith unless to are trying to make something unique or want to make a non-standard modification.
What happened to weapons in the Elite? Did the manufactures suddenly get blasted back to 1812? Why do these mass-produced weapons not have any kind of commonly sold attachments or modifications? Why do I have to gather a bunch of random matts like the engineer is some medieval blacksmith that's about to forge me the red-dot sight of high mithgaurd? Logically any manufacture who created any kind of modular system would become the de facto leader of handheld weapons in-universe if these ultra-specialized weapons are all that the current manufacturers pump out.
Like think about it, imagine if there was a ship manufacture in-game that only created ships for one specific role, so much so that they don't make their components swappable at all, they'd go out of business as soon as another manufacture put out a ship that could do what that specialized ship can do but also provide modularity. Most people would just go with what gives them more options and that'd make even more sense in terms of weapons in Elite, with a commander never really finding themselves in one environment for too long.
So yea thx for coming to my ted talk about how guns make no sense in the lore