[pornel]

That is the pitch, but not the reality.

C ABIs (of which there are many because the C standard doesn’t cover ABI) are full of legacy cruft, because they need to be stable and backwards-compatible more than they need to be sensible or efficient.

The C ABI can even vary depending on compiler flags, e.g. availability of AVX affects calling conventions. It’s not easy to be interoperable with this, especially when ABI-affecting compiler flags and macros may be set by an arbitrary build system, not even the C source code.