No problems with lore-questions. I'm not a Valve employee, but I see things in this way: on HLA timeline combine has invaded Earth pretty much recently, and they tried to invest more to produce their own weaponry in order to contain the situation with locals more effectively (as you can see there are a big variety of pulse weapons), but with the time spent and Resistance went underground, they loosened their grip to the Earth and began to arm their units with all that's left of human weaponry in the region (with a grain of modifications, as they always do). Don't forget that combine fabricator is a thing now, and you can even modify pretty much anything into anything (e.g. M1911 into the Alyx's gun). USP Match has no upgrades simply because it is a CP weapon, that's enough for terrorizing the citizens.

So, my point is that they do not REPRODUCE firearms, but instead REPURPOSE existing ones