What goes into the Expo SDK?
The simplest way to look at Expo's SDK is to think of it as a toolbox that Expo provides. 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 can't include each and every native module out there because that would make for an extremely heavy toolbox (i.e. large binary size and maintenance cost). Instead, we only include native modules that we believe satisfy a wide range of needs for a large majority of developers. If you need a specific tool that isn't provided in the Expo toolbox, then you can always use the bare workflow and add any native module you'd like to your project, while still enjoying most of the benefits that Expo provides.
If you think there is a module that does satisfy a wide range of needs for a majority of developers and isn't in the SDK yet, then we encourage you to open a feature request on our Canny so that we might include that module in the future!