Amazon is mostly drop shippers too, and Etsy for that matter, what's being sold by "shops" on Shopify or "merchants" on Amazon isn't really the issue. It should not at all be surprising that "low effort" shops are more numerous. There's quality stuff on all those platforms, and Shopify (for now) is better to the merchants.

Amazon's total dominance of online shopping is because of their logistics network and having more than one company providing fast inexpensive shipping is good, it means people have somewhere else to go if Amazon gives them a raw deal.