Hi Harper,
Thank you for the compliment. I think the short answer is that Shopify doesn't want to get into the app business. Social Login is a complex feature and to build it at scale for PaaS (platform as a service) makes it even more difficult. Every time you create a new app it needs to be supported, maintained and upgraded. This is tough to do when you're trying to focus on building the best e-commerce solution on the market.
Just my two cents.