Because the Chinese companies don't have the brand recognition. Most consumers would see a Chinese name and think "low-quality knock-off" and unable to distinguish a Chinese company trying to compete on quality versus one competing on price.
Probably because nobody got to them yet. Or because they haven't determined where the "sweet spot" is between quality and prices... yet.
I observed a progression going like this in my hobby (firearms and accessories):
1. No Chinese goods. Expensive US and European brands dominate the market.
2. Chinese goods enter by extending their airsoft product lines, and quickly dominate the "cheap junk" market. But there's a huge gap in both price and quality between them and older brands. Older brands still retain a large part of the market, because the quality of that new stuff below "good enough" in most cases.
3. Small companies that don't have an established brand name and compete on prices start outsourcing to China and offering products at "made-in-China, QC'd-in-USA" prices, with warranty, tech support etc. The prices are now about 1/3 of established brands, unlike the junk before that was at 1/20. This creates a whole new market segment, which takes away a lot of customers from those brands, because it's past the "good enough" bar.
4. Large companies jump on the bandwagon to compete. You start seeing essentially the same design with minor cosmetic changes being rebranded and sold by various companies. Because this is fairly obvious in the age of the Internet, the value of the brands diminishes.
5. Chinese suppliers find their way to the American market, and start selling the same stuff directly, offering comparable warranties and customer support, but at even lower prices. New brands get established in this manner (e.g. Holosun).
6. Chinese brands start targeting the high-end market, and adding exclusive features for their own products, that are not available in generic product lines intended for rebranding (at least, not until sometime later).
The industry was around #1 up until mid-2000s or so, and it just got to #6 about a year ago. Anything that's not made in China is squeezed into narrower and narrower niche at the top of the market. Holosun got to the point where they are making $1K+ electronics that is competing against $2K+ equivalents by established brands - in other words, they're already contesting for the part of the market way above where most customers are.
Slapping a random name on a cheap product from the Yiwu market[1] is far quicker and easier than building a brand. There are a lot of Chinese middlemen who have the skills to ship boxes profitably, but don't yet have the skills to establish a brand name in the Western market. Dollar stores and big-box retailers are full of cheap no-name products of indifferent quality, because there's a sustained market demand for literally the cheapest option.
Chinese manufacturers and retailers are moving up the value chain as fast as they can, but it's not an overnight process and no-brand goods aren't going away.
Because not all Chinese companies are the same? Some are selling non-brand or fake-brand on Amazon or Taobao, they might just be some factory in rural Guangdong or Zhejiang somewhere, not even a company. Some are actually trying to build a brand and are even controlling retail to avoid the hassles of middle men (Xiaomi).
My impression is that it is often a series of random companies. A "factory" doing some part, or assembly, might not be larger than a class room. These days you can pretty much just search for something like "tablet factory shenzhen" and see how it is [0]. Of course then there will be larger ones like Strange Parts visits [1], and sketchier ones [2]. And then on top of that will be all kinds of companies doing design, distribution, sales and whatnot.
For some reason many of them don't have much presence outside of China. (It is obviously not very hard to find Chinese branded products in China). Maybe the overhead or the cultural gap is just too big at the moment. I will however be interesting if they start selling internationally at scale.
