system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 4] Why are the guns left-handed?

I'm looking for a Watsonian *and* Doylist explanation. 

This always bothered me when playing Fallout 4. I did a Google search and none of the threads (mostly on Steam forums) offered anything worthwhile. 

I haven't found any reason for the manual-action guns in FO4 being left-handed, either in-universe or meta. The only consistent guess I've seen is that the developers wanted players to be able to see more of the guns' operation, which is reasonable with a bolt-action rifle being used in first person. But it's such a sharp contrast with real-life firearm operation that it sticks out to me.
gpt
Watsonian:

Fallout weapons are made to be easily swapped to be ambidextrous. 

Boston always had a large post-war population but relatively few guns (low guns per capita). After centuries of intensive use, the right-handed parts have mostly broken and not been replaced. So few of these parts are left that left-handed guns became the norm at some point, meaning even newly manufactured weapons are built left-handed because that's what everyone is used to using by now.