C doesn't have a standardized ABI, either, and yet it's the most widely used ABI in the world anyway.

Just because it isn't standardized doesn't mean it isn't stable & relied upon anyway. Clang & G++ are ABI compatible with each other, for example, and more importantly (and relevantly) both libstd++ & libcxx strive to maintain ABI compatibility ( https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html & https://libcxx.llvm.org/DesignDocs/ABIVersioning.html )