Many times I’ve seen sellers drive traffic to their Shopify and WooCommerce product pages with no buy or cart option. The only way to buy the product is a link to their Amazon listing.
Like you, I also wondered why the sellers aren’t driving traffic straight to Amazon but routing through their privately owned store when their only goal is to sell on Amazon. The short answer is that these sellers use Amazon as a cashier.
Since how well you rank on Amazon is determined by how high your conversion rate is, for external traffic it makes most sense to provide the product details and description outside of Amazon, and only take warm traffic to Amazon with high purchase intent.
This has a much better impact on your rankings in comparison to routing external traffic directly to your Amazon listing that may or may not convert.