One of HTTP's biggest strengths here are that it's essentially perfectly language-agnostic (in practice, not just theory) and it lets you solve basically all networking problems (hardware and software) in normal, off-the-shelf, trivial-through-Google-scale ways without having to change it. And if you want to build something custom, there are more experts and documentation and examples than any other system.

I agree it's suboptimal for performance. But it absolutely cannot be beat for business stability and flexibility. And your IPC costs generally do not dominate your request times anyway.