I have the feeling if reader macros were added to C today, most programmers would ban them from their projects because they'll be treated as C++ abominations that result in code that is too complex to debug, dealing with inline template/operator expansion in every potential line of code. Obviously C preprocessor cannot handle such 'power' and C lexer would not be able to determine the intent when syntax is dynamically altered by code.
One of C's strength is readability, and C code is obvious when you
read it(macros can be mentally expanded too with some effort).