Because it's cgo-free maybe?
-tags netgo,osusergo -linkmode external -extldflags -static
I regularly compile (cross-compile, even) static Go binaries that use the cgo sqlite package. But it's certainly a lot simpler if you can avoid cgo.
but the overheads also stack up, the database/sql api is fairly allocation heavy too unless you do a lot of work and that friction increases quite a bit with the ffi boundary.
this is not to suggest “modernc is faster” - it’s not for a lot a workloads.
there are opportunities for optimization all over both approaches.