Why are there no automatic theme updates in Shopify?
Themes are commonly customized heavily by their shop owners and there's no way to simply incorporate the theme updates around those customizations. Software can be updated automatically because users aren't able to modify the underlying code. When a software update is released, it knows exactly what existed before it and what's been changed since.
Themes, however, are entirely different. They're meant to be customized to suit your needs and reflect your brand. If Shopify were to run automatic updates on themes, the updated version wouldn't be able to tell what was original code from the version before it and what was added by the shop owner. It would have to overwrite everything and the shop owner would lose all of their hard work.