> Why can’t extensions be a standalone thing in the store?

1) Because then you need a whole parallel set of processes for configuring, updating, and uninstalling those things, distinct from the existing processes for apps. And you need to make that process accessible to users who may be used to everything being an app.

2) A nontrivial number of browser extensions on iOS are part of standalone apps anyway, like password managers or bookmarking tools. It'd be very strange to have both app-with-browser-extensions and browser-only-extensions, or to require some extensions to be installed and updated in tandem with a companion app for expected functionality.