The top tier makers don't "make all their own stuff."
Colt doesn't even forge - they contract the platters and machine. There are dozens of makers of the smaller parts, some obscure but major players. They supply the tiny parts the bigger players know aren't worth the hassle. Even the biggest players can't consume the production capacity of one spring wire forming machine working two shifts five days a week.
You can make small parts much more quickly than the big ones, what do you do with 2-3 years of production capacity in house? You either accept contracts for other stuff, like S&W does milling Harley transmission parts, or support your competitors? Not.
Just buy the springs and things from the big contract spring makers and be done with it. QC them on the dock before acceptance and move on. No capital investment amortizing another machine or having to keep an operator, supplies, etc. going all the time running it. You expense a parts order without all the overhead. All major corporations do that now. Even if they own the subsidiary, they make each one it's own profit center that has to bid the job. That way there's no corporate accounting that disguises how bad the plant is.
Having depth in sources is a strategic arms issue to DOD - they went so far as to set up LAR as an outsource for uppers. That got absorbed by Grizzly, who sold to "Remington." With FN and Remington both getting contracts there's no huge impact losing LAR.
Ruger might make all the parts for their own proprietary brand, but move into the AR world, not so much. Don't forget there are foreign plants across the world who insisted they get the machinery for their own production - same as the Pakistani's having their own G3 HK factory. Korea and Israel come to mind. That's why you don't see the little US flags on every item in the Brownell's catalog - they are being honest that a lot of gun parts for the AR are not US made.
It's just one of those things the American shooter doesn't want to know about, like all the budget 1911's coming from Turkey, the Czech's, or the Philippines - roll marked with some Big Name Brands at the factory.
It's all about outsourcing, it's not about making every spring and pin if you don't have to.