Aren't some of those platforms more-or-less official outlets of the manufacturer for some brands already?

While it's entirely possible some of the storefronts are just flashing "official widgetco shop" as a credibility-enhancing gesture, it's probably also the easiest way if you're a Chinese firm with little understanding of global last-mile logistics and small-dollar payment processing to get into the direct-to-consumer business. I thought AliExpress was spawned from the B2B relationships Alibaba already had.

If you put up a rule like that, I suspect those sites would just pivot to being "Shopify for Chinese Vendors" -- offering an embeddable storefront that the manufacturer can put directly on their page. The only losers would be the consumers, who would no longer get the convenience of centralized search, being able to put together an order from ten vendors in a single shopping cart, and the ability to efficiently combine shipping.

And let's not say "we lost manufacturing." We GAVE IT AWAY. It's not just that foreign labour is cheaper, it's that Asia was industrializing later, so you get state-of-the-science facilities, while the American plant is 50 years old and nobody wants to splash the capex to rebuild it to modern standards.

What you're saying is that American companies should be able to profit from the price disparity between China and US by reselling Chinese goods to US consumers at massively inflated prices, but regular Americans should not be able to do the same on their own.