Using the C preprocessor is standard, available, compatible and the major usage patterns are "known". For a lot of cases, they're way easier to reason about rather than learning how an external generation tool is used to generate the code. In order to understand these macros all I need is to read the source code where they're used.

Nothing C++ related in the pattern though. This C preprocessor trickery is practically so classic you couldn't necessarily even call it a "trick".