Complicated for the single-pass PDP-11 compiler. Everything else that can fit AST in RAM can do it relatively easily. Modern C compilers have millions of lines

Complicated for the single-pass PDP-11 compiler. Everything else that can fit AST in RAM can do it relatively easily. Modern C compilers have millions of lines and hundreds of person-years of work in them, so two passes and a lookup table is not a big deal. It's just not done because the C standard doesn't say it should. Perhaps there is some preprocessor crime that this isn't compatible with, or they just don't want to break that PDP-11 compatibility.