Shopify's moat is its ecosystem of developers and apps, for many of the same reasons WordPress still powers 39.5% of all websites in 2021. Shop Pay also provides many of the same benefits of Stripe -- global one-click payment across all their stores and more. I run into Shopify sites while shopping all the time and am always pleasantly surprised that I don't have to fill out anything to check out, it already knows me. Then those orders magically show up in the Shop app on my phone for order status and delivery tracking.
Here's a screenshot. I did not make an account with any of these completely independent websites. Yet I checked out on all of them without typing my address or payment info. And all my orders appear here with tracking.