For instance, if you search for mouse pad in the US you'll see the Amazon Choice is a third party product as is the Best Seller.
The Amazon Choice has this right under the price:
Note: Available at a lower price from other sellers, potentially without free Prime shipping.
When you click on it you see you can save $1.30 if you're willing to wait 3 weeks. To me, this is handled properly.
UK suppliers tend to ship same day or next day, with delivery within one to three working days.
If a supplier isn't overwhelmed with orders, the rational choice is to ship ASAP, because it increases positive feedback and the prospect of reorders.
Not all suppliers understand this, but most do.
You only have to wait weeks on orders shipped from abroad. But the product source country is usually obvious from the listing, so you can allow for that - if you want to.
Really now? Can you be that obtuse? It's to sell directly and make more profit than having a 3rd party sell an item.
Tl;Dr. Amazon prefers to return Amazon's stuff on top of searches. News at 11.