If you do that, you’re taking on a side job in Amazon sales. Not everybody has the time or resources to devote to that.
By all accounts, it’s a sucky job. To succeed in it, you have to invest either a bunch of time or a bunch of money – or both – in Amazon’s opaque make-your-product-visible-to-enough-people algorithm.
Plus, until you sell enough of your stock, you’re in the hole for it. Which is worse than a 1-to-10% affiliate deal.