They still have an internal ABI used when static linking; aligning structures, laying out data sections, function call behavior, and so forth. C's flexibility/ambivalence in this regard is why it's preferred on those architectures.
One could consider static linking in this scenario just an optimization for incremental builds with separate compilation of source files; it's not really used to, e.g. link a library that has no source. In that sense, it's just an implementation detail that's leaked from a build system that notionally recompiles from source but has unwisely exposed the units of its caching system.
Microcontroller programs are also pretty small; it's not difficult to load the whole program into memory on a workstation and compile the whole thing together. This is how Virgil (circa 2006) worked.
