They hand it over to them because of how Amazon operates.
Let’s say you’re a small shop and you ship out oven mitts. Paying for storage of your oven mitts can get expensive. So instead of paying for storage, you can actually have Amazon store them in their warehouse and allow people to buy directly from there. You don’t have to pay Amazon extra for storage, just their standard fees.
It does make sense if you have a business model that doesn’t support storing your products.
I don’t like it, but it makes sense for businesses.