Products can be defective or even harmful without you noticing.
Does your multi vitamin actually contain every vitamin it says it does in the dosage it should? For many things you really depend on reputation and outside regulations as you’re hardly going to send random samples to a lab.
Even reputable companies make use of this. Look into how often SSD internals get downgrading without changing the product’s label.
AliExpress's user interaction is different, and IMHO much less friendly.
In Amazon, the user interacts with a product, and from here can either allow it to be fulfilled from whatever's its default seller, or select one explicitly. In AliExpress, the user interacts with the [product,seller], so in search you'll generally see 50 instances of the same product, and leave it to the user to decide which one to open into Product Details. IMHO, the Amazon interaction is far superior. (that's not an excuse for them not to look for a solution)
I prefer the opposite. eBay and AliExpress are a better experience because I interact directly with the seller. The seller has control over their listing, their inventory, and their fulfillment. I'm not playing the lottery as with an Amazon listing.
"Ships from and Sold by Amazon" if you don't want foreign sellers.
That doesn't solve the problem, because it locks out legit Chinese sellers. Solving the problem is hard.