Because that's where the customers are. Above all the Prime customers. And customers can have a lot more trust in Amazon's shipping speed and returns policy, than when ordering from an unknown.

A ton of people, when they want to buy something, put it into Amazon's search box. If a product doesn't show up there, it might as well not exist.

Also, Amazon reviews. I can't count the number of times I've chosen not to buy a product because Amazon's reviews warned me away. Ordering something from a small shop where there are no reviews, or the reviews seem fake or too positive? I feel like there's a much greater chance of being taken advantage of with a shoddy product.

End of the day, on Amazon I generally know exactly the quality I should expect, that it will arrive on time, and that I can return it if there are any issues.