One reason could be that most real-world compiler/runtime codebases are in C++ (gcc, llvm/clang, v8, most of Java JITs) and C++ has just enough high-level featu

One reason could be that most real-world compiler/runtime codebases are in C++ (gcc, llvm/clang, v8, most of Java JITs) and C++ has just enough high-level features to make a large code base somewhat manageable. So, if one wants to work on a industrial-strength compiler at some point, it might be not the most expressive and helpful choice, but it makes sense.