Slight tangent: a long time ago when I was into this hobby, Oshpark was usually the go-to instead of JLCPCB. What happened to them? Macrofab too if anyone's used them.
There are US shops like Advanced Circuits that can turn boards around extremely quickly, but obviously you are paying for that speed. Most people ordering a handful of boards for a hobby project, or prototype are probably optimizing more for cost and consistency than shaving a few days off the lead time.
Once someone finds a vendor where they know the quality will meet whatever their own subjective minimum is, there's not much incentive to shop around. Being able to tack on assembly at JLC with a single checkbox also exponentially increases the desire to just stick with that vendor.