Most of the Shopify plugins exist to give a store owner an advantage in sales, conversion or marketing. Sure execution counts, external branding counts, but Shopify is a closed platform. You use their plugins and approved apps. And most store owners rely on these plugins to some degree, to help them build brand awareness etc.
Eg: https://apps.shopify.com/browse/sales-and-conversion-optimiz...
Shopify levels the playing field which is great. But if everyone selling Bananas is on the same field, using the same plugins, switching fields could be a great way to differentiate and possibly optimize beyond what Shopify can help you with.
There also are a number of apps who choose not to be in the app store and not give up 20% of their revenue and instead market on their own.
Shopify doesn't make advantages like branding, brand awareness, marketing, market research, etc available. It simply makes setting up an online store more convenient for people do. Convenience != competitive advantage. You can extend this argument to cloud platforms. AWS, and Azure make setting up and scaling a SaaS easier, but it doesn't give you any secret sauce that you need to succeed, like the ability to know what your customer want.