This explanation makes the most sense to me, but there is something that you are leaving out. Not only is Amazon a marketplace, but it is THE marketplace. Amazon has an effective monopoly on small-seller logistics and marketplace services in the United States (and many other places), soup-to-nuts.
If you are anything other than a massive corporation, any manufacturer that chooses not to sell through Amazon and utilize all or most of its services (marketplace listing, payments, warehousing, delivery) will be at a massive cost disadvantage and will not be able to compete with other sellers that do choose to participate with Amazon.
And more significantly, perhaps, if you don't sell through Amazon's marketplace, you are often unable to compete with Amazon itself.