Earlier quoted context omitted.
Well, Amazon/Ebay/Shopify/Etsy all are platforms that let other people sell things. I get your point (and might even agree with you), however if you look at it from the perspective of "I want to sell something... where do I go?", they're competition. They do discovery, checkout, inventory, fulfillment, etc. It just happens to have a slightly different approach than the others.
Not really the same thing though, Amazon/eBay/Etsy you can browse multiple "stores". Shopify is just a platform so it really isn't that fair. It'd be like including every single Magento powered site to say Magento is the X largest platform.
Adding search across stores = centralized.