The reason they don't do this is a legal one. As a platform, they are not liable for what their sellers sell (copyright/trademark/counterfeit), but as a marketplace they would have deeper liability. A central marketplace also gives brands the ability to search through all Shopify stores, basically giving them a search engine to find infringement.
How do I know this? 10 years in ecommerce with a different ecomm platform.
How do I know this? 10 years in ecommerce with a different ecomm platform.