What's the value of having sqlx in the stdlib? The only reason I could see is a social one, not a technical one (discoverability).
The Go devs did in the past include external libraries in the stdlib, the most significant one being "golang.org/x/context" moving to "context". This was a very important inclusion because it meant the stdlib could start offering context-aware APIs, and it was a signal to the rest of the ecosystem that the context API is stable forever and other packages can make their APIs context-aware without risk of future breakage. With sqlx, none of these issues arise: the main compatibility interfaces between different SQL libraries (particularly to the drivers providing support for different RDBMS) are already present in the stdlib.
The Go devs did in the past include external libraries in the stdlib, the most significant one being "golang.org/x/context" moving to "context". This was a very important inclusion because it meant the stdlib could start offering context-aware APIs, and it was a signal to the rest of the ecosystem that the context API is stable forever and other packages can make their APIs context-aware without risk of future breakage. With sqlx, none of these issues arise: the main compatibility interfaces between different SQL libraries (particularly to the drivers providing support for different RDBMS) are already present in the stdlib.