The simplest way to look at Expo's SDK is to think of it as a toolbox that Expo provides in Expo Go, and that can be used in part or in whole in any React Native app. Included in that toolbox are what we believe are the most essential and widely usable tools for building extraordinary and powerful apps (like a hammer, powerdrill, and saw). You can do a lot with just these tools and creative thinking. Of course, there are other tools out there that are made for narrower use-cases (like a tailpipe cutter). These can be really useful, and sometimes are even make-or-break for specific apps.
We only include native modules in the Expo SDK if we believe they satisfy a wide range of needs, or if we think that integrating them deeply into Expo tools and services will provide additional value. If you need a specific tool that isn't provided in the Expo SDK, which is a very common requirement, then it's time to migrate from Expo Go to development builds. Learn more about adding custom native code