So, Shopify does have something similar called the shop.app which is only for Shopify stores. My best guess is that the e-commerce platforms are solving a different problem: store creation. I'll also note that each individual e-commerce platform isn't incentivized to aggregate and send users to stores not built with their platform. When we launched in December, we only had Shopify stores. Now we have Shopify and WooCommerce, and working on support for Squarespace and Wix sites as well.
For user traffic, the primary strategy is to build features with a viral loop back to the product. I mentioned this in another comment but we have have the concept of making shareable 'lists' of products you like. This is already working for us, at a small scale.
The general plan is to aggregate as many e-commerce products as possible from different platforms, keep improving the search experience, add automated filters to ensure high quality products, and then keep layering in features that drive a viral loop back to Agora.